has descriptions for every Linux flavor out there, as well as the mirrors to download them. Just make sure the file size downloaded is the same as what is listed. I have gotten files that were too small before.
Red Hat - Easy to use and highly developed. Bloated and will bring an older computer to it's knees (also depends on packages you choose). Intended for corporate use. Uses RPM's for installs.
Mandrake - The "other" easy to use distro. More for home use than corporate. Uses RPM's for installs.
Debian - Most support and packages available. Also one of the most Unix like. Uses apt-get to install files and handle dependencies.
Slackware - The other most Unix like, and also the most stable platform. Uses pkgtool for install, but mostly you'll be compiling from .tgz files yourself adn handling your own dependencies, which can get old quick. But you will learn a lot more a lot faster.
Gentoo - Newer on the scene and highly touted by users. Eses Emerge (similar to Debian's apt-get) to install and handle dependencies. Also has the hardest installation of all time! No GUI whatsoever. I had a laptop displaying the installation instructions next to me and it still didn't work for me. The good part is performance, which is probably the best of any distro. Gentoo is optimized for YOUR system (PII, PIII, PIV, Athlon, or whatever) and compiled when you install (the reason it takes so long to install).
College Linux - Haven't tried yet, but it is based on the solid Slackware distro, adn is meant for ease of use for College students new to Linux.
Other people have other opinions on the 200+ other distros. For learning, Knoppix is extremely popular because it is a LiveCD. So you load the CD on boot and it runs a full Linux install at runtime, but doesn't actually install anything. This is great for rescueing Windows and also for learning Linux on a PC you may not own (or want to tamper with).
Personally, I use Slackware at home, but I am only running a PII350! So RH and Mandrake are out for me.
Sean.