Uniquely San Francisco experiences?

I love all that has been mentioned so far, but here is one that is very unique to San Francisco. It is called Friday night skate. At 9pm every Friday down by the ferry building you will see a ton of people with Rollerblades or skates. They start at the ferry building, go down the Embarkadero, to Fort Mason, though 2 tunnels and then at a Bart station, down escalators, then back to the ferry building. This skating event ends around midnight. What a sight to see.