Well, I've heard that Ubuntu is a Great Operating System, and I'd LOVE to try it. I have a MacBook, and don't want to replace OS X Mountain Lion. How do try Ubuntu without uninstalling OS X?
You can use VirtualBox for OSX
This will let you run a copy of Ubuntu in a window on your Mac. You could also shrink your OSX partition and use ReFit, but if this is your first time with Ubuntu and you want to just try it, I would stick with VirtualBox (or, if you have it, VMWare)
Instructions for shrinking your OSX disk space to make room for Ubuntu and doing a dual boot install are here: https://help.ubuntu.com/community/MactelSupportTeam/AppleIntelInstallation
But note the comment about using this on recent macs and the latest Ubuntu