Ubuntu is a hugely favoured OS in the open source community, it's also completely free to use. I've used it on desktops and have liked it a lot. It's just unfortunate there isn't too much software support for it.
Canonical, the people behind Ubuntu made a huge announcement in London last week - they will be releasing a mobile version of the OS.
Even though the OS is not complete, it looks promising to most. It uses many gestures to help you get around the OS, instead of using buttons. So it does take getting used to. It has some unique features like the notification bar, where if you swipe down from an icon, it'll show options for that.
Canonical have confirmed that the phone version of the OS will be available for Galaxy Nexus owners in February. It won't be a finished product, but something users can play around with.