Flutterby™! : Mapnificent

Interesting: Mapnificent lets you pick a point, and then munge a slider to show you the region accessible via public transit within the slider selected time from that point.

Big holes in the Bay Area, it apparently just knows about Muni, BART and a little bit of CalTrain (and knows nothing of the Golden Gate Transportation District, AC Transit or SamTrans), but an interesting use of, presumably, the General Transit Feed Specification, and a cool way to look at data.