Alas, San Fran is not much of a happy hour town compared with DC and other cities.
It also depends on the day of the week. Location is also a factor - as it could be quite a walk to some of these places if you are south of market or north of montgomery.

Thurs/Fri might find a large crowd at the Irish Bank, particularly if the weather is good. Also Kell's. Ditto for Gordon Biersch and Palomino if you are south of market - also the Thirsty Bear. MoMo's is packed when the Giants are playing.

I would also second dhuey's recommendations, with the exception of the overpriced Enrico's tourist trap.