The City of San Francisco has recently developed a new website (http://datasf.org) to help disseminate data related to the city’s elections, environment, geography, health, housing, public safety, public works, and transportation. Many of which are available in GIS format!
From the site:
DataSF is a clearinghouse of datasets available from the City & County of San Francisco. While there is plenty of room for improvement, our goal in releasing this site is:
(1) improve access to data
(2) help our community create innovative apps
(3) understand what datasets you'd like to see
(4) get feedback on the quality of our datasets.
In addition to the wealth of data sources, there is also a section showing off some of the applications that outside developers have created with the data.