If you're a fan of arcade games, and are willing to put in a little time compiling a program to run it, then MAME4ALL is right up your alley. Franxis has released the source code for versions 4.6 and 4.7. These optimized versions of MAME, is based on the gpx2 port and offers a better compatibility (more games supported) than previous versions.

If you're new to the whole emulation scene, MAME stands for Multiple Arcade Machine Emulator. Designed to emulate the hardware of arcade games, it was first publicly released on February 5, 1997 by Nicola Salmoria and the rest of the MAME team with an open-source license.

Installation isn't very complicated, but you will need GCC 4.0.2: DevKitGP2X rc2 to compile the code or a slightly modified version of GCC 4.1.0 (DevKitARM r18) linked with GCC 4.0.2 (DevKitGP2X rc2). Before doing anything go over the Read Me document included in the compressed file to avoid potentially damaging your PSPs.

Thanks to Francisco Vergara for the tip!