GNU-Darwin has gotten support for AMD's Opteron processors. "Mac OS X users are welcome to give free software a try now. I finally got a chance to try GNU-Darwin-1.1 on an Opteron machine with Darwin-7.2. Testing with glxgears confirms that the MesaGL is performing about twice as fast as on a comparable 32-bit CPU, although some other tasks are clearly not yet optimized. The machines are otherwise roughly identical. Here is a screenshot."