If you’re interested in learning about Linux you should know about DistroWatch. They keep a Page Hit Ranking table on their homepage, which allows one to easily see which distributions are trending in the Linux community, and one can get a pretty complete thumbnail sketch of what a distribution is about with just one click. Download mirrors, related websites, screenshots & reviews are all posted on the information page for any distribution, so any thirst you might have for more information is easily slaked. DistroWatch is to Linux as IMDB is to movies. It’s good stuff, and a great resource.
Browsing DistroWatch makes me wish I had more free time to play around with these various distros. If you’ve got the bug and are ready to play, but would rather not screw around with (OK, screw up) your main PC, consider creating a virtual machine with something like VMWare Player or VirtualPC. Then install the distribution of your choice to that and you’re ready to explore a new OS.