I love the concept of Trapster, but it’s a long way from actually being useful.
I do love the Spotify app.
Another app that I've re-started using again recently -- and which appears to be very popular in my area -- is Waze (also owned now by Google).
It incorporates a lot of the Trapster features (plus those of GasBuddy) and is a crowd-sourced traffic app. At any given time on the road it claims to have a couple doxzen other users in my immediate area, reporting on traffic conditions in realtime.