For the really not faint at heart, Canonical has made available a developer preview of the upcoming Ubuntu Touch operating system. This developer preview is just that--a preview for developers. It's still quite rough around the edges, much more so than, say, the Windows 8 alpha or beta.
If you aren't scared off by the possibility of bricking your device, then you can consider installing the developer preview on your Galaxy Nexus, Nexus 4, Nexus 7, or Nexus 10. You'll need to know what you're doing to attempt this and TweakTown takes no responsibility if something happens to your device. Make sure your device is one of the supported ones at Ubuntu's Touch website.
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