I just started West Coast Swing classes at this place in Sunnyvale on Monday nights: http://www.steppinoutproductions.com/
Beginner classes start at 8:30 pm and cost $10. There's a dance after (which is quite fun and included with the lesson). The teacher is fairly good (a marvelous dancer). The beginner classes go for 5 week sessions (but if you know something about dancing, you can drop in any week); at some point, you can move up to the intermediate class which is much bigger.