We're not certain if you've heard about this one already before but a couple of the many projects of homebrew developer kgsws are PSP Doom and PSP Heretic. It is needless to say that these are PlayStation Portable adaptations of the FPS classics Doom and Heretic.

Currently, PSP Doom is at version 0.6 while PSP Heretic is at v0.7. kgsws mentioned , and we agree, that the best feature these build have to offer is the multiplayer ad hoc for two players. The coder also added a four player support but that hasn't been tested as of yet. The following changes have occurred since the last versions.

PSP Doom v 0.6 and PSP Heretic v0.7:

* Sound Volume: its value is controlled by distance of its source.
* Puff Bug (engine bug): puff does not appear when projectile (bullet, not rocket etc) hits two-sided wall under sky.
* Weapon selecting: old weapons selecting was making multiplayer out of sync (and heretic does not have any switcher)
* Sky colormap (engine bug?): when in GOD mode (no cheat) everything except sky was in different color

PSP Heretic v0.7:

* Added weapon switching
* Fixed sound bug of imp (that red fling monster)
* Added cheat menu in singleplayer
* Added CPU settings in options