SNES9x is a SNES emulator developed for the PC the Unofficial SNES9x for PSP is an unofficial port of the emulator for PSP. It seems to have to most features of all the available NES emulators. This homebrew is frequently updated, and there are few reported problems from users. It runs well, with most games playing at their full intended speed.
NesterJ is the most-used, and most-likes NES emulator for PSP.