Hi - take a look at

• the Website of the San Francisco Folk Music Club, http://sffmc.org/.
• You really should experience a song circle at Faith Petric's house in San Francisco - they're held every two weeks (info at sffmc.org).
• There's also a wonderful chantey sing on the first Saturday of the month at the National Maritime Museum at Hyde Street Pier (free- reservations required), http://www.nps.gov/safr/.
• My favorite folk venue in the San Franciso Bay area is The Freight and Salvage in Berkeley, http://www.thefreight.org/