Mostly the larger, multi-issue environmental organizations. More specialized ones are listed under their area of focus.
Audubon Online - National Audubon Society
http://www.audubon.org/
The Audubon Society, founded in 1905, is probably one of the oldest conservation organizations around. Since it was named for John James Audubon, who is primarily known as an ornithologist, there is a fair bit of focus on birds, but many other species and issues are also discussed, both on the site and in their print magazine.
Canadian Parks and Wilderness Society
http://www.cpaws.org/
One of the largest nationwide wilderness organizations in Canada. News, action alerts, links to local chapters, and current campaigns, including two international, multi-organizational initiatives they participate in, the Algonquin to Adirondacks (A2A) and Yellowstone to Yukon (Y2Y) projects.
Defenders Of Wildlife
http://www.defenders.org/
A longstanding wildlife conservation group, focussed primarily on the US. News, publications, and special sections on specific animals such as wolves, bats and big cats.
Earth Island Institute
http://www.earthisland.org/
The organization founded by environmental pioneer David Brower, after he parted ways with first the Sierra Club and then Friends of the Earth. EII aims to "provide activists the freedom to develop program ideas, supported by services to help them pursue those ideas, with a minimum of bureaucracy." Deals with a wide variety of issues, from predator conservation in North America to indigenous people's rights in Borneo.
Friends of the Earth International
http://www.foei.org/
The world's largest federation of environmental groups, with over one million members in 66 countries, working on a wide variety of issues. Includes an archive of articles from their quarterly journal Link, a monthly electronic bulletin, list of member groups worldwide, and more.
Greenpeace International
http://www.greenpeace.org/international/
Not necessarily the largest environmental organization, but probably the best known. Paul Watson calls them "The Fuller Brush men of the environment" for their door-to-door canvassing techniques, and they've received a lot of flak over their level of bureaucracy, but they still do some pretty cool stuff.
Rainforest Action Network
http://www.ran.org/
This site is beautifully designed and packed with useful information. One particularly nice feature is an "Activist Toolbox" with tips for activist groups on how to effectively fundraise, write press releases, organize actions, etc.
Sierra Club
http://www.sierraclub.org/
Another very long-running organization, founded in 1892 by John Muir. Historical information on Muir and the organization, current campaigns, Sierra magazine, nature trips, and a store with a wide variety of books and other merchandise.
Wildlands League
http://www.wildlandsleague.org/
An Ontario wilderness protection group, affiliated with CPAWS. Information on Ontario wilderness issues, action alerts with the option to send automated messages to decision-makers on various issues, a wide variety of publications in PDF form, and more.
