PSP Quake 1.1 by Juraj Styk if you haven't played it. A (very) near perfect port of GL Quake to PSP.
IMO the best FPS on the system, commercial or homebrew (I guess it itself is a mixture of both).
There is a
version of PSP quake 1.0 for slim that can make use of the slim's extra RAM to allow it to play really big multiplayer maps, so i guess it's not 1.5 only though...
The whole singleplayer game and both of the expansion packs run great on my fat.
If you like FPSs, give it a try.
I cannot say enough good things about this port.