The "PRO" CFW for PSP claims to be "most compatible Aftermarket Custom Firmware". Please be aware that modifying your Firmware might brick your PSP and will break you warranty! Read more about PRO-B8 Custom Firmware (PSP OS)
CubicVR is a 3D Engine, also working under Dreamcast and Playstation Portable. If you are into 3D coding, you might want to check CubicVR out! Read more about CubicVR (misc)
CubeMania by Sand3r is a PSP homebrew application featuring 3 small puzzle games, called FillUp, Matrix and Slide. They're all quite similar to each other as they have the same design and idea: a game board that's made out of cubes, basically just squares. Read more about CubeMania v1.4 (PSP Game)
Kyle Luge comes up with his first full blown Playstation Portable homebrew game named TimeCube. It features multiplayer adhoc competitive play and single player modes! Read more about TimeCube v2.0 (PSP Game)
Dave Gnukem is a 2D scrolling platform shoot-em-up. It is very similar to and inspired by Duke Nukem 1. The initial port was done by deniska in 2006. It seems a user named lukensteinz has done a little bit of code cleaning, while also getting it to work on newer firmwares, Read more about Dave Gnukem (24-06-2011) (PSP Game Port)