Hopstop covers subways and buses in San Francisco. GPS predicts time of the next bus.
NextBus uses GPS to tell you the predicted time of the next bus. Google maps show buses in real time, and you can get updates on your phone/PDA. The coverage is limited to certain agencies within the US, so these other sites might be useful: Hopstop covers subways and buses in NYC, Boston, Chicago, San Francisco, DC, and more. (mobile version) Google Transit has many US metro areas in addition to Canada, Europe, and Japan. Many more locations inside.