If you're feeling experimental and you're willing to get your hands dirty to be on the bleeding edge, there's a version of Ubuntu available that will run on the Android Nexus 7 tablet. Canonical is looking for developers and testers to give it a whirl and make sure it works well.
To do this, you'll need to replace Android, so make sure you back up all of your data before you start the process. Once you're ready to go, grab the tablet-friendly build, a USB cable, and follow the instructions. The process is pretty straightforward, although it does involve unlocking your Nexus 7's bootloader first.
If you're a developer, Canonical is looking for your help ironing out bugs and optimising Ubuntu for use on tablets. You can read more about the project at the Ubuntu wiki, or at the link on Ubuntu's community manager blog below.