Septerra Core is REALLY not steampunk.... maybe cyberpunktargren said:Torchlight - Steampunk Diablo
Septerra Core - Legacy of the Creator (Steampunk RPG)
(Both for PC, but I think TL is available on XBLA...)
Thief is a mixture of fantasy and steampunk and one of the best games ever made. Not kidding, it's awesome. It quite literally invented the stealth genre and the first 2 games are amazing, the 3rd game is pretty good.
Arcanum is also pretty nice, very open and complex RPG for it's day, but unfortunately flawed.
I think some of the Final Fantasy games are steampunk (don't ask me what games exactly)
Syberia is a pretty well known and well received steampunk adventure game