The Alpha Xenon Development Kit 

When Microsoft was developing the Xbox 360, it turned to the PowerPC processor to power the console. To allow developers to work on software before the exact chip Microsoft wanted was ready, it turned to the Power Mac G5. The company used the original dual 2 GHz model, as well as the updated 2004 variant of the tower, and wrote a bunch of custom software for it, including a complete operating system.

Pierre Dandumont has written a lot more about this topic:

Then, Microsoft buys many Power Mac G5 and installs a dedicated operating system on it for the devs. Before the arrival of the real console, the brand provides two variants to the developers: the Alpha 1 and the Alpha 2 kits. As we will see, they differ essentially by the graphics card used. The development kits are used during a few years, at least until the E3 2005: the demonstrations of the Xbox 360 at the time are on Mac. Then, Microsoft downgraded the machines, which were eventually sold to employees for $ 300, with obviously a completely erased hard drive. From time to time, those machines reappear on eBay or at private homes – I have one – but obviously without the Microsoft OS (and with Mac OS X).

Turns out, this software recently showed up online:

The software can be downloaded from this webpage, which links to a wiki dedicated to this odd machine.