Like the Web itself, this page will always a work in progress. I've collected below links to websites that seem to me especially interesting and helpful for the various subjects on which I work, with special emphasis on environmental history, historical geography, and environmental studies generally.
These are in no particular order, but I've tried in general to place the broadest and most comprehensive sites toward the top of these lists. It should probably go without saying that I don't necessarily agree with or vouch for the content of all of these sites, but I have generally found them richer and more useful than many others on the Web.
Please contact me if you think others should be added, or if you find broken links that I've not yet discovered.
American Society for Environmental History: http://www.aseh.net/
Center for Culture, History, and Environment of the UW-Madison Gaylord Nelson Institute for Environmental Studies: http://envhist.wisc.edu
Environment and History: http://www.whpress.co.uk/?domain=www.erica.demon.co.uk
H-Environment: http://www.h-net.org/~environ/
European Society for Environmental History: http://eseh.org/
Forest History Society: http://www.foresthistory.org/
Environmental History Resources: http://www.eh-resources.org/
Carolyn Merchant's Environmental History: Explore the Field: http://nature.berkeley.edu/departments/espm/env-hist/eh-internet.html
Environmental History Virtual Library: http://vlib.iue.it/history/topical/environmental.html
Environmental Protection Agency Envirofacts Data Warehouse: http://www.epa.gov/enviro/
Environmental Protection Agency History: http://www.epa.gov/history/
U.S. National Park Service History: http://www.nps.gov/history/
U.S. Forest Service History: http://www.foresthistory.org/Research/usfscoll/index.html
U.S. Fish & Wildlife Service History: https://nctc.fws.gov/history/index.html
Harvard Forest: http://harvardforest.fas.harvard.edu/
Arnold Arboretum of Harvard University: http://arboretum.harvard.edu
Library of Congress American Memory Site on Evolution of Early Conservation Movement: http://memory.loc.gov/ammem/amrvhtml/conshome.html
Spencer Weart's Discovery of Global Warming: http://www.aip.org/history/climate/
St. Andrews University Research Centre for Environmental History and Policy: http://www.cehp.stir.ac.uk
Life and Legacy of Rachel Carson: http://www.rachelcarson.org
American Historical Association: http://www.historians.org/
Organization of American Historians: http://www.oah.org/
Canadian Historical Association: http://www.cha-shc.ca/
National Council on Public History: http://ncph.org/
Library of Congress: http://www.loc.gov
National Archives: http://www.archives.gov/
Center for History and New Media: http://chnm.gmu.edu/
History News Network: http://historynewsnetwork.org/
Digital History: A Guide: http://chnm.gmu.edu/digitalhistory/
Library of Congress American Memory Website: http://memory.loc.gov/ammem/index.html
University of Kansas Digital History Website: http://www.digitalhistory.uh.edu/images.cfm
Yale University Senior Essay Handbook: http://www.yale.edu/history/senior_essay.html
Zachary Schrag's Guidelines for History Students: https://zacharyschrag.com/ (look under Guidelines for History Students)
Statistical Abstracts of the United States: http://www.census.gov/prod/www/abs/statab.html
Census On-Line: http://www.census-online.com/
Ancestry.com: http://www.ancestry.com/
Cyndi's List of Genealogy Sites on the Internet: http://www.cyndislist.com/
Uchronia Alternate History List: http://www.uchronia.net/
Association of American Geographers: http://www.aag.org/
United States Geological Survey: http://www.usgs.gov/
U.K. Ordinance Survey: http://www.ordnancesurvey.co.uk
U.S. National Atlas: http://www.nationalatlas.gov/
U.S. National Atlas of 1970: http://memory.loc.gov/ammem/gmdhtml/census3.html
NASA Visible Earth: http://visibleearth.nasa.gov/
Louis J. Maher's Geology by Lightplane: http://www.geology.wisc.edu/~maher/air.html
USDA Natural Resources Conservation Service Soil Maps: http://www.nrcs.usda.gov/wps/portal/nrcs/site/soils/home/
National Geographic Society: http://www.nationalgeographic.com/
Royal Geographical Society: http://www.rgs.org
ESRI GIS and Mapping Software: http://www.esri.com/
Journal of Historical Geography: http://www.elsevier.com/wps/find/journaldescription.cws_home/622880/description#description
Historical Geography: https://ejournals.unm.edu/index.php/historicalgeography
Google Earth: http://earth.google.com/
Google Maps: http://maps.google.com/
Microsoft Live Search Maps: http://maps.live.com/
Wikimapia: http://www.wikimapia.org
U.S. Census Bureau: http://www.census.gov/
U.S. Census Bureau American Factfinder: http://factfinder.census.gov/
CIA World Factbook: https://www.cia.gov/library/publications/the-world-factbook/index.html
American Planning Association: http://www.planning.org/
U.S. Board on Geographic Names: http://geonames.usgs.gov/redirect.html
U.S. Census Bureau Gazeteer: http://www.census.gov/cgi-bin/gazetteer
U.S. Census County and City Data Book: http://www.census.gov/statab/www/ccdb.html
Place Names: http://www.placenames.com/
Dave Leip's Atlas of U.S. Presidential Elections: http://www.uselectionatlas.org/
Library of Congress Panoramic Maps or Birds'-Eye Views: http://memory.loc.gov/ammem/awhhtml/awgmd7/panoramic.html
David Rumsey Map Collection: http://www.davidrumsey.com/
University of Texas Perry-Castañeda Library Map Collection: http://www.lib.utexas.edu/maps/histus.html
Central Pacific Railroad Photographic History Museum: http://cprr.org/Museum/Maps/
Europe Maps: http://www.euratlas.com/
Open History European Historical Atlas: http://www.openhistory.net/
Oddens' Bookmarks, "The Fascinating World of Maps and Mapping": http://ncph.org/
epodunk: http://www.epodunk.com/
Sperling's Best Places: http://www.bestplaces.net/
Environmental Directory: http://www.webdirectory.com/
Envirolink: http://www.envirolink.org/
Conservation Directory (National Wildlife Federation): http://www.nwf.org/conservationDirectory/
Grist: http://www.grist.org/
Wilderness Society: http://wilderness.org/
Trust for Public Land: http://www.tpl.org
Conservation Campaign: http://www.conservationcampaign.org/
Land Trust Alliance: http://www.landtrustalliance.org/
Sierra Club: http://www.sierraclub.org/
Nature Conservancy: http://www.nature.org/
Natural Resources Defense Council: http://www.nrdc.org/
Ecological Society of America: http://www.esa.org/
Environmental Careers Organization: http://www.eco.org
Student Conservation Association: http://www.thesca.org/
Environmental Leadership Program: http://www.elpnet.org
Orion Magazine: http://www.orionmagazine.org/
Wilderness.net: http://www.wilderness.net/
Wildlands Project: http://www.wildlandsproject.org
Urban Wildlands Group: http://www.urbanwildlands.org/
Urban Land Institute: http://www.uli.org
Cultural Landscape Foundation: http://www.tclf.org/
Center for Land Use Interpretation: http://www.clui.org/
Conabio: http://www.conabio.gob.mx/
Scorecard (find out toxic releases in any zip code): http://scorecard.goodguide.com/
Intergovernmental Panel on Climate Change: http://www.ipcc.ch/
Climate Ark: http://www.climateark.org/
RealClimate: http://www.realclimate.org/
Cryosphere Today (real-time views of polar ice caps): http://arctic.atmos.uiuc.edu/cryosphere/
Alternative Technology Association: http://www.ata.org.au/
Centre for Alternative Technology: http://www.cat.org.uk
Project Gutenberg: http://www.gutenberg.org/
Project Gutenberg Australia (has books still under copyright in U.S.): http://gutenberg.net.au/
Archive.org: http://www.archive.org
Google Book Search: http://books.google.com/
MobileRead News (best source on mobile eBook technologies): http://www.mobileread.com/
AudioBooksForFree.com: http://www.audiobooksforfree.com
University of Virginia Digital Collections: http://lib.virginia.edu/digital/collections/
University of Pennsylvania Online Books Page: http://onlinebooks.library.upenn.edu/
University of Michigan Making of America Digital Library: http://quod.lib.umich.edu/m/moagrp/
Interuniversity Consortium for Political and Social Research: http://www.icpsr.umich.edu/
Internet Public Library: http://www.ipl.org/
ArchiveGrid: https://beta.worldcat.org/archivegrid/
Artchive: http://artchive.com
Bartleby Great Books Online: http://www.bartleby.com/
Ebooknet: http://www.cnomy.com
Memoware eBooks for PDAs: http://www.memoware.com/
University of Wisconsin Digital Collections: http://uwdc.library.wisc.edu
Great Books and Classics: http://www.grtbooks.com/
Historical Text Archive: http://historicaltextarchive.com/
bibliomania: http://www.bibliomania.com/
Religion-Online.org: http://www.religion-online.org/
Web of American Transcendentalism: http://www.vcu.edu/engweb/transcendentalism/
100 Milestone Documents of American History: http://www.ourdocuments.gov
Chronology of U.S. Historical Documents: http://www.law.ou.edu/hist/
Library of Economics and Liberty: http://www.econlib.org/
William Blake Archive: http://www.blakearchive.org
Dime Novels and Penny Dreadfuls: http://web.stanford.edu/dept/SUL/library/prod/depts/dp/pennies/home.html
Ralph Waldo Emerson Complete Works: http://www.rwe.org/
William Faulkner: http://www.mcsr.olemiss.edu/~egjbp/faulkner/faulkner.html
Grand Canyon Bibliography: http://www.grandcanyonbiblio.org/
William James Writings: http://www.des.emory.edu/mfp/james.html
Marxists Internet Archive: http://www.marxists.org/
John Muir Writings: http://www.yosemite.ca.us/john_muir_writings/
Native American Resources on the Internet: http://www.hanksville.org/NAresources/
Edgar Allan Poe Writings: http://www.eapoe.org/works/index.htm
Thoreau Reader: http://thoreau.eserver.org/
Popular Photography: http://www.popphoto.com/
Steve Digicams Reviews: http://www.steves-digicams.com/
Norman Koran Digital Photography Tutorials: http://www.normankoren.com/index.html
Imaging Resource: http://www.imaging-resource.com/
Digital Camera Resource Page: http://www.dcresource.com/
Picnik Online Photo Editing Tools: http://www.picnik.com/
A Few Scanning Tips (wonderful basic guide to scanning techniques): http://www.scantips.com/
Center for Culture, History, and Environment of the UW-Madison Gaylord Nelson Institute for Environmental Studies: http://envhist.wisc.edu
UW-Madison Department of History: http://history.wisc.edu/
UW-Madison Department of Geography: http://www.geography.wisc.edu/
UW-Madison Nelson Institute for Environmental Studies: http://nelson.wisc.edu/
Wisconsin Historical Society: http://www.wisconsinhistory.org/
Turning Points in Wisconsin History: http://www.wisconsinhistory.org/turningpoints/
Dictionary of Wisconsin History: http://www.wisconsinhistory.org/Content.aspx?dsNav=N:1575
Wisconsin Geological & Natural History Survey: http://wgnhs.uwex.edu
Thematic Maps of Wisconsin: http://www.uwex.edu/wgnhs/sample.htm
University of Wisconsin Digital Collections: http://uwdc.library.wisc.edu/index.shtml
Wisconsin Department of Natural Resources: http://www.dnr.state.wi.us/
UW-Madison Environmental Remote Sensing Center: http://www.ersc.wisc.edu/
Great Lakes Information Network: http://www.great-lakes.net/
Great Lakes Environmental Atlas: http://www.epa.gov/glnpo/atlas/index.html
Minnesota Historical Society: http://www.mnhs.org
Chicago Historical Society: http://www.chicagohistory.org/
Encyclopedia of Chicago: http://www.encyclopedia.chicagohistory.org/
UW-Madison Campus Map: http://map.wisc.edu/
UW-Madison Lakeshore Nature Preserve: http://www.lakeshorepreserve.wisc.edu/
Cultural Landscape of the UW-Madison Campus: https://uwdc.library.wisc.edu/collections/uw/uwmadison/uwculturalland/
Introduction to the Public Land Survey System: http://www.sco.wisc.edu/plss/public-land-survey-system.html
Wisconsin Public Land Survey Maps: http://digicoll.library.wisc.edu/SurveyNotes/
Wisconsin Land Economic Inventory Maps (1930s): https://uwdc.library.wisc.edu/collections/EcoNatRes/WILandInv/
Wisconsin Blue Books: https://uwdc.library.wisc.edu/collections/WI/WIBlueBks/
Wisconsin Ecology and Natural Resources Collection: http://digicoll.library.wisc.edu/EcoNatRes/
Wisflora: Wisconsin Vascular Plant Series: http://www.botany.wisc.edu/wisflora/
Chronicle of Higher Education (you have to subscribe, but a great resource): http://chronicle.com/
Beloit College Mindset Lists: https://www.beloit.edu/mindset/
Louis Shmier's Complete Random Thoughts about Teaching: http://www.halcyon.com/arborhts/louis.html
Refdesk: http://www.refdesk.com/
Wikipedia: http://www.wikipedia.org/
Internet Public Library: http://www.ipl.org/
Merriam-Webster Online: http://www.m-w.com/
UW-Madison Writing Center Writer's Handbook: http://www.wisc.edu/writing/Handbook/index.html
UW-Madison Writing Center Website: http://www.wisc.edu/writing/
Online Etymology Dictionary: http://www.etymonline.com/
Visual Thesaurus: http://www.visualthesaurus.com
How Stuff Works: http://www.howstuffworks.com/
YourDictionary: http://www.yourdictionary.com/
USA.gov (comprehensive portal to U.S. government information sources): http://www.usa.gov/
Myth and Metaphor: http://www.mythosandlogos.com/mythandmetaphor.html
Internet Movie Database: http://www.imdb.com/
Biographical Directory of the United States Congress: http://bioguide.congress.gov
Behind the Name (Etymology and History of First Names): http://www.behindthename.com/
Catholic Encyclopedia: http://www.newadvent.org/cathen/index.html
New Yorker Cartoon Bank: http://www.cartoonbank.com/
Bartlett's Quotations, 1919 Edition: http://www.bartleby.com/100/
Quoteland: http://www.quoteland.com/
Quotation Reference: http://www.quotationreference.com/
Quote Garden: http://www.quotegarden.com
Baby Name Wizard: http://www.babynamewizard.com/
David Allen's Getting Things Done: http://gettingthingsdone.com/
ChemiCool Periodic Table of the Elements: http://www.chemicool.com/
Online Conversions: http://www.onlineconversion.com/
Merck Manual of Diagnosis and Therapy: http://www.merck.com/pubs/mmanual/
Human Anatomy Online: http://www.innerbody.com/htm/body.html
Everyrule.com: http://www.everyrule.com/
Top Secret Recipes: http://www.topsecretrecipes.com
UW-Madison Lakeshore Nature Preserve: http://www.lakeshorepreserve.wisc.edu/
Anne Spirn's West Philadelphia Project: http://web.mit.edu/wplp/index.html
Monticello Explorer: http://www.monticello.org/
Language of the Land: http://www.loc.gov/exhibits/land/
Built St. Louis: http://www.builtstlouis.net/
Manhattan Timeformations: http://www.skyscraper.org/timeformations/intro.html
Photosynth demo: http://www.ted.com/talks/blaise_aguera_y_arcas_demos_photosynth
NASA Earth Observatory (great recent satellite images): http://earthobservatory.nasa.gov/
Grist: http://www.grist.org/
Internet Scout Project (weekly review of great websites, many of them environmental): http://scout.wisc.edu/
Chronicle of Higher Education (you have to subscribe, but a great resource): http://chronicle.com/
History News Network: http://hnn.us/