MIT Shuttle Bus Map - this suggestion came in as a comment post on the UofM bus map I mentioned yesterday. In the comment Quentin Smith pointed out that "MIT has been doing this (bus mapping) for several years with ShuttleTrack. It's a bit tricky to enable Google maps - visit the prefs to toggle the interface from the static maps to Google Maps.
Other Boston Transit Maps Mashups:
Boston "T" Subway Map - One of the original Google Maps transit mashups. Shows the entire T subway map.
Boston Subway Map Tools - Find the nearest subway to your location, shortest direction between two stations, or the fastest way between two points using the subway.
Is there a Google Maps transit mashup for your city? If not, make one! :)