Up until now, most native iPhone games, excepting some great emulators (NES and Gameboy Advance), have been less than entertaining. There are two relative newcomers, however, that beyond showing off the iPhone's touchscreen and accelerometer potential for gaming are actually fun.
The first is iBlackJack. All you need to know about this game, aside from it being blackjack, are: double-tap to hit, swipe down to double down, swipe right to stand. It works like a charm, and has some of the most polished graphics we've seen yet in any native iPhone application. If only this game were linked to an online gambling site; we could be losing hundreds from anywhere!
The second is a Spy Hunter clone called "ContraSense". All you need to know about this game: It's Spy Hunter, and you tilt the iPhone to steer. This title, though very simple, gets kudos for its excellent use of the accelerometer and genuinely fun gameplay.
Both of these titles are available under the "Games" section in Installer.app. See our Applications page for instructions on using Installer and enabling your third-party native applications on your iPhone.