Great oysters for Happy Hour from 5-7pm M-Sat and 4-7pm Sun at Mecca on Market Street between Dolores and Church Streets (towards the Castro). They also have awesome $6 cocktails to go with their $1 oysters. Paul is one of the coolest bartenders there. Very friendly but semi-upscale ambiance. It's definitely a great place to hang out, "in cognito".
There's also a new place that just opened up on Market and 14th Streets, Woodhouse Fish Company. They have $1 oysters on Tuesdays from 5pm-closing and $7 dungeness cold crab.
Another place for Happy Hour $1 oysters towards Russian Hill is the Hyde Street Seafood House on Hyde and Jackson Streets. Fabulous oysters at $1 during happy hour, 7 days a week.