A homebrew classic, dating back to early 2010, is Codetactics' Mobile Assault. The only pimped-out, homebrew helicopter shooter for your PSP ... and your iPhone too ... if you're sporting one of those (check the AppStore for it).
Mobile Assault is like Jungle Strike, and you remember how awesome that was, right? Beef that up and you got Mobile Assault v1.8.1. And since we missed version 1.8 I can tell you it adds a full French and partial Italian translation ... and a "really bad" partial translation into Spanish. So calling out all Spanish-speaking PSP sceners... Lend a hand! Visit the source and get in contact with Codetactics. Translate that!
You'll also find other improvements and bug fixes in 1.8 and 1.8.1.