We're all anxiously awaiting the launch of the iTunes App Store in early July. That said, there're plenty of great jailbreak iPhone games out there. The one I've been most enjoying of late is Passionfools' title CubicMan (video link). A remake of the Flash game Bloxorz, CubicMan presents the user with an isometric game grid and the challenge of manipulating a rectangular object in such a way as to stand it up on a designated grid square, thereby moving to the next level.
The game is great fun and makes good use of the iPhone's accelerometer for screen orientation and multitouch display for pinch-and-zoom playfield manipulation.
CubicMan can be had via Installer.app from the ModMyiFone repository for users of a jailbroken iPhone or iPod touch. We hope to see this available through the iTunes App Store at launch.