This weekend is the annual SF Bike Expo at the Cow Palace, for all things bike. Watch some crazy BMX and fixed gear demos, see local designers (such as the gang from Nooworks) showcase their newest products at the Pedal Savvy fashion show. For the first time, the Expo is going to be two days instead of one, so you have more time to check out the giant swap hall or learn to go build your own bike.
You can buy tickets at Pushbike, or check out the SF Bike Expo website for tickets and more details. If you want to ride down there (which you should!), there will be a group ride leaving from Mission Bicycle both mornings. The shindig starts at 10am both days.