NextBus has really improved my experience of transit.
Thanks to alphacygni, who pointed me at NextBus, my experience of the MBTA bus service is greatly improved. NextBus supplies real-time arrival information using GPS data supplied by transit authorities. You can retrieve the information via the web, SMS, or voice call.
I've found it particularly useful now that it's cold out.
This is a system that is more useful in places where the headways can be long.