Volunteer.gov: Federal clearinghouse of volunteer opportunities.

State Parks: Information on volunteer positions for California's State Park System.

BLM: The Bureau of Land Management has varied and interesting volunteer opportunities.

National Parks: One of the most popular selection of volunteer opportunities, particularly for interpretive positions.

Dept. of Interior: Clearing house of volunteer opportunities for a variety of federal agencies.

Take Pride in America: A national partnership program aimed at increasing volunteer service on America's public lands.