Windows 95 put onto iPhone 3G
This is a bit silly, but quite impressive. Someone's managed to cram Microsoft's Windows 95 onto an iPhone, and get it working via the touchscreen, going as far as to play a quick game of Minesweeper.
The performance isn't the snappiest, thanks to the emulator used - Bochs Emulator, if you're curious. The team tried Windows XP too, but didn't seem to have much luck.
If you could run any operating system from history on your handset, which would you pick and why? Answers in the comments.