Senor Quack released the source code of his Abuse port for the GP2x. Abuse is a famous side-scrolling run and gun game originally released for DOS and Linux in 1996. It features beautiful pixel art, lighting effects, many weapons, and is a real challenge even on normal difficulty.

http://archive.gp2x.de/cgi-bin/cfiles.cgi?0,0,0,0,46,2688