You know what always cheers me up? Making things do stuff they shouldn't.
Next week my class are going to play around with Ubuntu 10.04. First we're going to do a fresh install, then we're going to test some terminal commands and then... we're going to get root.
Earlier this week, I installed Ubuntu 10.04-3, which unfortunately comes pre-patched. Today I installed Ubuntu 10.04-netbook which comes with an earlier kernel (hopefully the one we're going to use). Open source is great, the apparent downside is that exploits are rather easy to find (or check the patch notes and write your own exploit). Since I'm new, I downloaded one of exploit-db, compiled it, and rooted my virtual machine.
Of course there are other ways to get full access to an Ubuntu machine. Booting via a live cd and then modifying sudoers might work, or perhaps bribing the admin.