dr_watson has released a great looking Space Invador Clone for the PSP, heres the info from the Release Thread
"This is a prototype of StarBugz, a Space Invader clone. The purpose of the project was to get myself familiar with programming PSP games, and hopefully to build up a simple game engine in the process.
After working on the project in my spare time for nearly a month, the hardware accelerated 2D game engine, called JGE (Jas Game Engine), was basically done. JGE uses Gu libraries for image rendering and StarBugz now becomes a small showcase of the engine, demonstrating various features of it. "