ESXi 4.1 on a Mac Pro
I retired my Mac Pro (the Mac Pro 1,1, or the model from 2006. More info available at http://www.everymac.com/systems/apple/mac_pro/stats/mac-pro-quad-2.66-specs.html), and decided to see how it would run ESXi.
A default install of ESXi 4.1 worked beautifully on the machine with no customization. It recognized both NICs, and saw the storage. A Mac Pro could make a decent server for someone for home or non-production use since it is fairly quiet and has 4 drive bays. I imagine the newer Mac Pros probably won't have the necessary driver support to run ESXi, but then again it'd be a strange choice of machine to run it.
There were a couple of annoyances, namely that you can't open and close the optical drive once the machine has booted into ESXi since it requires that you run Mac OS X (or Windows with the appropriate BootCamp drivers).