I don't know why, but all the commercials I have seen for GTA IV have made me really interested in the game. Theres something about the commercials that I can't quite put my finger on that makes them really appealing, I think their song choice is a big part of it, somehow it fits. I bring this up because I just saw the newish one for Episodes from Liberty City and now I want to play them.
New one
http://www.youtube.com/watch?v=ZSNlzWSQxOk
Older ones,
http://www.youtube.com/watch?v=ZVZ8b75w6vM&feature=related
http://www.youtube.com/watch?v=47EWHY3E5AM