As Melissa mentioned we visited the Cable Car Museum on Saturday. It was relatively small and easy to take in, but quite fascinating at the same time. The fairly primitive technology behind the cable cars is remarkably clever.
There are three operating cable car routes in San Francisco (California, Powell-Hyde, & Powell-Mason), but there are four separate lines (California, Powell, Hyde, & Mason). The museum lies roughly in the middle of these four lines. In addition to the cable car museum, this building also houses the motors and pulleys that actually operate the cable cars.