Rides app: casual carpool (with a possible extension into an anti-DUI app)
First phase of this app would allow people to log in (using FB authetication, Google, Twitter, etc.) and share location based info--people checked into a location as a driver or rider would display numbers of riders or drivers at a given location.
As people became more engaged, the app could be expanded to include comments, feedback, and to eventually assist riders in finding rides back from SF.
If the model was successful enough (with enough users and level of community/trust, this same app could be extended to help people find rides home at other times (think late night after Bart has closed).