Darwin is the core operating system that lies under both Mac OS X and iPhone OS. It is the true core foundation that bridges the kernel to the actual UI above. (SpringBoard/loginwindow/etc).
With the help of @plus_chan and his Nokia N900, I present to you, Darwin/ARM running on a Nokia N900.
I doubt this has any usefulness whatsoever, but that doesn't make it any less awesome. Great work (via Steven Troughton-Smith).