Clarion Alley is totally worth a visit when you're in San Francisco. It's right in the middle of the Mission near a bunch of great places to eat and drink. Balmy Alley is a 10 minute walk away and another great street art gallery.

Clarion Alley Mural Project does great work maintaining Clarion Alley. (Including relations with property owners - this problem is an anomaly, there's been art there successfully for over 30 years.) Also worth knowing about Precita Eyes, a local community organization that manages murals in a lot of the city.