In the South Bay, I find two places that are legal, and I even enjoy in my odd way, are the fire roads of Sierra Azul (Woods, Kennedy, Limekiln,...) and the zillions of miles of fire roads of Henry Coe. At Henry Coe you even get multiple stream crossings through in for good measure. Bring wool socks.
I compiled a list from comments on last year's MTBR of the NorCal spots good for rain riding. If you're willing to travel, some singletrack can be found. This may be my excuse to try areas like Angwin, Rockhill and Red Hills, normally so far I haven't yet rode them.
- Sierra Azul (Los Gatos, Kennedy area) Fire Roads
- Henry Coe (East of Gilroy) Fire Roads only in rain, scattered Singletrack when dry
- Demo AKA Soquel Demonstration Forest (Santa Cruz/San Jose) Does OK I hear, but better to wait a couple days after rain
- Angwin & Boggs & Oak Hill (40 min north of Napa) Singletrack
- Red Hills (Central Valley, Sierra Foothills, 20 min east of Oakdale) Singletrack
- Rockville (Napa) Singletrack
- Annadel (Santa Rosa) Mostly Singletrack, some Fire Road (Wins my award so far as most fun)
- China Camp (San Rafael) Singletrack
- Fort Ord (Monterey) Fire Road and Singletrack mix
- Tamarancho (Marin) Singletrack
- Wilder (Santa Cruz) Some reports say too muddy??