Leslie - there are some great jams in the area; unfortunately NOT in S.F. itself. There are 3 places in SF you can count on hearing good blues almost any night: Biscuits and Blues http://www.biscuitsandblues.com/, Lou's Pier 47 http://www.louspier47.com/cgi-local/sche.cgi and The Saloon http://www.sfblues.net/Saloon/Saloonschedpage.html. I only know of one jam in SF - on Tuesday nights at Skip's Tavern http://www.skipstavern.com/ - pretty much acoustic, can go from sort of OK to awful.
However there are excellent jams in the bay area. Worth it even if you have to rent a car. At the top of the list would be RJ Mischo's jam at Mojo Lounge in Fremont http://www.themojolounge.net/ on Tuesday nights (45 minutes from SF) and Wednesday night at the Redwood City Blues Jam at The Little Fox Theatre http://www.rwcbluesjam.com/ (35 minutes from SF) Both of these draw nice crowds and excellent musicians. There is also a new jam in Pacifica on some Thursday nights. On Sunday afternoons there is a good jam at The Pioneer Saloon in Woodside http://pioneer-saloon.com/ and another good one Sunday eve out on the coast at Old Princeton Landing http://www.oplmusic.net/
The best resource is the Yahoo Group "BayAreaBlues" - http://launch.groups.yahoo.com/group/bayareablues/messages - there is a calendar there that is reasonably accurate and any jam that actually happens is very likely to be on it, as well as every blues show within 100 miles or so.
Hope that helps
/Greg Heumann http://www.blowsmeaway.com http://www.bluestateband.net