King County Metro (Seattle) had a system like this back around 2003. Interestingly enough they did not have GPS on the buses, they'd simply instrumented the trip meter which was plenty good enough since buses were (usually) running along fixed routes. The real time location of the bus was displayed in a fairly crummy Java applet, but at least for the route I live on it worked great. I just waited for a bus to show up on the map about a mile from my apartment and that gave me plenty of time to walk to the nearest the bus stop.

I have found One Bus Away to be completely inferior, to the point that I usually don't bother to check it.