Warning: some NSFW lyrics on the music in the video above.
What do you get when you combine an old Xbox with OS X and some Mac Pro level hardware? This monstrosity. Built by Will Urbina, the OS Xbox Pro is a hackintosh casemodded into an original Xbox dev kit, with some crazy hardware under the hood, including a pair of 2.93GHz Intel Core 2 Quad Q9550s, an NVIDIA GeForce 9800GT card, 8GB of RAM, an 16GB solid state drive, and four traditional hard drives -- one boots Windows 7, the other one does OS X (Snow Leopard, we believe -- he bought a copy retail), a Ubuntu install in there somewhere, and two other sweet hard drives for video editing.
And here's the wacky thing: Total material cost of the system is under $1500. That, combined with the video above, almost makes me want to spend a month of weekends trying to do something like this myself (aside from, you know, fitting it into a dev kit Xbox thing -- although fitting it into an old Macintosh might be a fun build, too). At any rate, awesome build for sure.