A guy in Australia has used Debian and an emulator to put OS X on Centris 650, an ancient 68040-based Mac (25Mhz!) with only 68MB of RAM.

Then at 9pm on Monday 25th October 2004, a Mac Centris 650 started booting OS X. About an hour and a half later, the familiar Apple logo bootsplash appeared What next?
I Wait. According to the developers, PearPC using generic emulation (the only option on a non-x86) runs “about 500 times slower” than the host CPU. Ouch. That makes for a 0.05MHz G3, at the best. That’s around 4000 times slower than the Athlon boots, and since that takes roughly 2 and a half minutes – I’m looking at at least 6.99 days. One week to boot!

Fantastic.