In addition to the previously mentioned apps/games, here's some more. Grab 'em while they're free. Or grab them later when they're not because they're still quality games:
- Mirror's Edge™ for iPhone, Mirror's Edge™ for iPad - I played this on Xbox 360 and it's great fun. Not sure how well it translates to a smaller screen.
- Flight Control for iPhone, Flight Control HD for iPad - Tell planes, helicopters and boats where to land before they crash.
- Peggle for iPhone, Peggle HD for iPad
- Dead Space™ for iPhone, Dead Space™ for iPad - Creepy looking first person shooter that I don't really want my kids to accidentally play.