In Oakland there is Sea Music every Thursday night from 8-10, no cover , with Skip Henderson and the Starboard Watch. For a real neighborhood jam, I've heard that the Bluegrass Jams at MCGraths pub in Alameda are great, but I have not attended one of their many session for at least a year.
There is also a Sunday afternoon Irish Session followed by an evening American Jam. Speisekammer Restaurant in Alameda also has bluegrass, old timey, tin pan alley, and other interesting genres of music on weekend nights.

Ceolas website will give you some info on Irish/Celtic sessions around the Bay Area but some of the info may be out of date.

There are numerous BlueGrass venues in San Francisco once again as well as some Old Timey jams here and there.

I think there is also a Pizza place in Pinole with bluegrass most nights.

Alameda Elks Lodge had a well known Cajun/Zydeco dance every Friday night. Top Zydeco groups make appearences here on a regular basis. Its a small venue. I think lessons are at 8 and the open dance starts at 9.

If you hang out around outside Dave and Pam Swans nieghborhood your also likely to hear someone tearing out a tonsil. :o, ;)

Gary