How cool would it be to be able to play old Nintendo or Sega games on your PSP? Well, if you can find the game ROMs, you can play them, thanks to the PlayStation Portable homebrew
community. I've picked what seems to be the best or most popular emulator for each of ten systems that can be emulated on the PSP. (Please note that these are links to exteral sites and are not official Sony products -- using them may void your warranty.)
NesterJ is the most-used, and most-likes NES emulator for PSP. It runs well, with most games playing at their full intended speed. This homebrew is frequently updated, and there are few reported problems from users. It seems to have to most features of all the available NES emulators.
SNES9x is a SNES emulator developed for the PC; the Unofficial SNES9x for PSP is an unofficial port of the emulator for PSP. Of the available SNES emulators, this one has the least amount of frameskip when running games at full speed. It is the most frequently updated and has the most available options.
It's not actually finished yet, but it looks like Monkey 64 is going to be a great emulator. Considering that most of the homebrew community didn't think there'd ever be a working N64 emulator for PSP, the progress on this one has been impressive.
Rin is an emulator for both Gameboy and Gameboy color (which makes sense, as the GBC could also play older Gameboy games). It seems to handle just about every GB and GBC game without problems, and it's got some nice features.
PSPGBA is a nice-looking Gameboy Advance emulator. It has multiple languages to choose from, and can be adjusted to boost the speed for some games that may run slow.
PSP Genesis is a fast Sega Genesis emulator, able to run most games at full speed. It also has lots of features and can play most Sega Genesis games without problems.
StellaPSP is a port of the Stella Atari 2600 emulator. A big advantage of Atari emulation is that there are quite a few game ROMs that can be downloaded legally for free. While the latest version of Stella isn't perfect yet, it does work, and a new version is on the way.
C64PSP is a PSP port of the Frodo C64 emulator. It's stable and runs games at full speed without problems. It's got some great features and the latest version even seems to have some compatibility with firmware version 2.0.
It's not perfect, but NGPSP does run some NeoGeo Pocket games without too many problems. It's the only PSP NeoGeo Pocket emulator out there, so if you want to play NGP games on your PlayStation Portable, this is what you need.
This emulator has lots of options and, while it still has bugs, there are frequent updates as the homebrewer works out the problems, and many games are quite playable.