What I love about San Francisco is that art abounds on the street. If you just walk around the city (and keep your eyes open), you are bound to find different kinds of art at every other corner.
Apparently, only the Mission District contains around 200 murals. Many of them were painted in the 1970s as part of the public program to provide space for young artist to express themselves. But SF mural art history goes as much back as to the 1930s when President Roosvelt's New Deal program funded series of murals that are now on display inside Coit Tower. SF boasts also three murals by Diego Rivera (located at City College and San Francisco Art Institute) painted in the 1930s and '40s.
Personally, I like a lot Clarion Alley Mural Project located on Clarion Alley, between 17th and 18th Streets, between Mission and Valencia. What is particularly nice about it is that every year they have a "block party" during which some of the old murals get replaced with the new ones.
Here are few pictures from Clarion Alley that I took during the walk with Ania and (the other ;-)) Monika last Monday afternoon (17th of June 2007):