Jpcsp

From the Jpcsp Google Code page:

Jpcsp is the most advanced PlayStation Portable emulator, allowing you to play your PSP games on a PC.

Jpcsp has been started in July 2008 and is developed by a small but active team.

Even though Jpcsp is written in Java, it can already reach 100% PSP speed on a lot of commercial games... and the emulator performance is constantly increasing.

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Download page