Golden Gate National Parks Conservancy –www.parksconservancy.org
The Golden Gate National Parks Conservancy has a wide variety of opportunities working in the Golden Gate National Recreation Area. They have both on-going and drop-in projects. Examples of projects include working at a native plant nursery, doing habitat restoration at ocean side and creek sites, and working at Crissy Field. Their web site has a lot of information on the different options.
Muir Woods. Crissy Field. Alcatraz. The Marin Headlands. Stinson 
Beach. Lands End. The Presidio. And 30 more beloved national park sites north and south of the Golden Gate.
Only one non-profit organization supports them all. The Golden Gate National Parks Conservancy.
Since 1981, partnering with the National Park Service and Presidio Trust, the Conservancy has provided over $500 million in support to the Golden Gate National Parks, rallied more than 250,000 volunteers, and pioneered innovative park stewardship and education programs.
Athenian Community Service Blog 