The US National Park Service Official website of the National Park Service. This excellent site offers printable park maps, general information on fees and camping and links to each park's homepage.  

USDA Forest Service Headquarters An excellent guide to the US Forest Service and Grasslands. Areas are listed by name, region and interactive map. Links to homepages of each National Forest. 

US National Wildlife Refuges Division of the US Fish and Wildlife Service. Searchable by region, this site has links to most of the National Wildlife Refuges in the United States. 

National Parks Worldwide Hosted by the GORP website. An interesting and useful listing of many National Parks throughout the world including Africa, Australia, New Zealand, Asia, Canada, Latin America and Europe.