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Cxbx is the most advanced Xbox emulator out there, which isn't saying much. It was started by Caustik and is kinda being continued by Blueshogun. There are currently a total of 5 games that are playable, according to an outdated compatibility list.
For those wondering why an Xbox emulator isn't already near perfect and assume that it's PC likeness should make it easy to write an emulator for, you should read this post by Blueshogun.