Burnout Paradise. If you told me that I would still be playing this game after 1 year I would not believe you. Its $20 now and I have played it for over 50 hours. I don't remember the last time I played a racing game this much.
Burnout Paradise, mostly for developer post launch support (DLC) that doesn't shaft gamers. Some paid stuff coming out now but they've given us loads of free new content already.