Sutro Project / Cable Cars
San Francisco's most iconic transportation. Definitely worth a ride if you can get one easily - waiting for an hour or more makes the decision harder. It's often easier to catch them at the non-terminal stops where everyone lines up. Look for the Cable Car stop signs.
For those interested learning more, there is a very nice (and free!) Cable Car Museum.
Website: http://www.sfcablecar.com/
Wikipedia: http://en.wikipedia.org/wiki/San_Francisco_cable_car_system
