Organization	Employees	Members	* Aspen Survey
National Wildlife Federation		4 M †	12
Sierra Club		1.3 M	64
World Wildlife Fund		1.2 M	5
Nature Conservancy	3,000	1 M	3
Natural Resources Defense Council		400 K - 1 M	20
Audubon Society		600,000	5
Wilderness Society		320,000	6
Environmental Defense Fund (EDF)		400,000	23
Environmental Information Center			4
League of Conservation Voters			3
The Environmental Working Group (EWG)			2
Greenpeace		2.8 M Supporters	1
Worldwatch Institute	28
Earth First
EarthJustice - Law firm affiliated with the Sierra Club
Defenders of Wildlife
ConservAmerica - GOP efforts in conservation
GreenFaith Leadership Programs � GreenFaith
General Consumer Advocacy Organizations
New Jersey Public Interest Research Group (NJPIRG)		25,000
New Jersey Citizen Action		60,000

See also New Jersey Environmental Organizations.
* 1998 Aspen Institute Survey: All the members of Congress and 29 top officials from the White House and federal agencies were asked to "Name the [two] national nonprofit organizations that you believe have the most influence on federal policy" in each of six issue areas. On environmental issues, the survey named the Sierra Club and the National Federation of Independent Businesses (NFIB) as the two most influential organizations, with the Sierra Club receiving more than double the number of responses as NFIB.

† You are a member of the National Wildlife Federation if you subscribe to one of their publicataions: Wildlife, Ranger Rick (ages 7 and up), Your Big Backyard (ages 3-7), ...

Others listed at Envirolinks.

There are many other specialized conservation groups such as American Rivers, American Forests, the American Farmland Trust, Ocean Conservancy, Chesapeake Bay Foundation, UN Development Program, and the Save-the-Redwoods League.
Ducks Unlimited, made up of 700,000 hunters, and the Izaak Walton League, made up of 50,000 anglers, are two of the largest groups working for wetland and stream protection.

The environmental movement does have radicals engaging in "direct action"--sitting for months in the tops of redwoods or driving nails into trees to frustrate the chainsaws, burning down the occasional ski lodge, tossing blood onto the furs of movie stars and socialites, or engaging in other acts of "ecotage." PETA (People for the Ethical Treatment of Animals), Earth First, and Greenpeace are among the most militant. A web search for "PETA and terrorist" came up with 20,000 hits.. See ecoterrorism at the Terrorism page.

Links:
A August 2008 article at SOURCE WATCH says:

Environmental activists and authors including Sharon Beder, Mark Dowie, Peter Montague, Brian Tokar, John Stauber and others have for decades criticized Big Green (Teh biggest environmental organizations in the United States) for soaking up the majority of the hundreds of millions of US dollars raised and spent each year on environmental activism, education and lobbying; raised for often abandoning or undercutting grassroots environmental struggles for fundamental change; and for often selling out the environment and the grassroots movement through business partnerships and agreements with compromising politicians.
...
With the exception of the Sierra Club, these groups have no meaningful accountability to the thousands of individual small donors who constitute their marketing lists and who are labeled 'members.'