\m/ Maybe this is old news, but I just discovered it, and I am really tempted to leave work early so I can download it and play it!
What is it, you ask?
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Frets on Fire, the open source Guitar Hero clone for the Windows, Linux, OSX, and FreeBSD!
Watch the
demo video. Looks pretty sweet. You can even use your Guitar Hero controller with it if you have a USB adapter. You can import the songs directly from your GHI and GHII discs. And there is evidently a huge user community that has created hundreds of songs. Wow.