Here are a few non-governmental conservation organizations. They are primarily wildlife-oriented because they are what I'm most familiar with.
Environmental Organization Web Directory
General Environmental Conservations Groups
Union of Concerned Scientists (UCS) is a non-profit alliance of scientists and citizens working for a healthy environment and a safe world, UCS researches and promotes clean energy and transportation technologies.
The Nature Conservancy :The mission of The Nature Conservancy is to preserve the plants, animals and natural communities that represent the diversity of life on Earth by protecting the lands and waters they need to survive.
Wildlife Conservation Groups
Izzak Walton League of America was founded in 1922 as a national organization of hunters, anglers and other conservation-minded outdoor enthusiasts who work through volunteer, community-based action and education programs to ensure the sustainable use of America’s natural resources.
The National Wildlife Federation is the nation's largest member-supported conservation group, uniting individuals, organizations, businesses and government to protect wildlife, wild places, and the environment.
The Wildlife Society, founded in 1937, is an international non-profit scientific and educational association dedicated to excellence in wildlife stewardship through science and education.
The Theodore Roosevelt Conservation Alliance
is a conservation association of fish, wildlife and habitat stakeholders who will motivate hunters and anglers to positively influence the future of our 192 million-acre National Forest system.
Hornocker Wildlife Institute has followed the wisdom of the great conservationist Leopold by using hands-on field research to understand our place and responsibility in the natural world's fragile ecological structures.
Groups dedicated to a species.
Quality Deer Management Association
National Wild Turkey Federation
Other groups of specific focus
Florida Native Plant Society was organized to promote the preservation, conservation, and restoration of the native plants and native plant communities of Florida.
Predator Conservation Alliance
(PCA) is dedicated to conserving, protecting, and restoring native predators and their
habitats in the Northern Rockies and High Plains.
Tall Timbers Research Station looks at fire ecology and related topics.
The Heritage and Wildlife
Conservation Council has been formed as a non-profit organization by private landowners throughout Georgia to work with state and federal agencies and wildlife organizations, such as the Georgia Wildlife Federation and Quail Unlimited, to develop a
reasonable and flexible approach to wildlife management.
The history of the Boone and Crockett Club is a chronicle of over 100 years of measured and thoughtful commitment to conservation. It is a commitment that balances human needs with wildlife needs; a commitment that sees deep value in preserving the hunting tradition, as well as conserving wild lands and wildlife; a commitment that grows out of a powerful love of wildlife.
The Cradle of Forestry in America commemorates the first forestry school in America, set in the mountains of North Carolina.