One of the city's historic and iconic San Francisco Cable Cars, which have been running since 1873 - the fare is $5 per ride, or you can buy an unlimited day pass for $11. The city has three cable car routes (here's a good map), all of them traversing the downtown area, and running either north or west from there. They don't reach the city's main neighborhoods of gay interest (well, although the California line does extend fairly close to the Hayes Valley/Civic Center neighborhood, which has a bit of a gay scene, and can be used to reach Polk Street, home to a few old-school gay bars).
If you're trying to get from downtown to either North Beach (which the Powell/Mason Line skirts) or Fisherman's Wharf and Russian Hill (which the Powell/Hyde Line leads to), cable cars are a fun and scenic (if touristy) way to go.