ItsYourDistrict is a site that allows you to search for community service opportunities in San Francisco.
The Bay Area Volunteer Information Center has a "search by interest" option as well as being able to search by geographic area (Peninsula/North Bay/etc.)
San Francisco Bay Area Volunteer Information Center lists lots of volunteer opportunities. Click here to see their list for San Francisco and San Mateo counties.
The Volunteer Center has a list of opportunities, called the Reach Out Guide, specifically for young people. You can alsosearch their database of volunteer opportunities using criteria like location, age, days of week, and others.
Volunteer Match lets you search for opportunities based on distance from your home, interest, and age.