Android for Windows phones -- simple concept, simple enough installation, but awesome results. The good people behind the XDAndroid project have been working hard to allow you to get your Google juices flowing nice and freely on your WinMo device and the latest build looks to have all but completed the task. Demonstrated on a Touch Pro2
-- a phone that recently got itself Ubuntu-ized
-- the Android installation experiences no difficulty in making calls, sending SMS or email missives, or browsing the web. There are still limitations, mind you, with GPS, Bluetooth and "other key functions" not yet available, but for the most part you're looking at the full Android experience on devices that weren't initially meant for it. Check it out on video after the break or hit the source link for detailed instructions on how to load this up on your own phone.
[Thanks to everyone who sent this in]Update:
Seems the Touch Pro2's keyboardless cousin has no intention of getting left out of the party -- m8cool has a little exposé
on HTC's Touch Diamond2
dual-booting WinMo with Android. Thanks, stagueve!