You can download a bunch of Linux distributions from http://www.linuxiso.org. They can be very tricky to install, though. SuSE is the easiest to install, and most complete distro. Red Hat Fedora and Mandriva are also good. Debian and Slackware for intended for experienced Linux users. Gentoo is pretty cool. And SimplyMEPIS is an up and comer. I do recommend using a Live CD distribution first, such as Knoppix because you can try it without actually installing it.
Looking for an easy to install Linux