First, understand that there are not that many "original" distributions - the ones all the others are based on. These include Gentoo, Debian, Slackware, Red Hat/Fedora, Suse, and Linux from Scratch. Each of these have their own derivatives for example:
Gentoo --> Sabayon, SystemRescueCdI suggest downloading and trying out each of these main distros. They all have a notorious reputation of being difficult to install. I have no computer science background and when I was just starting out, I successfully installed these 3 distros mentioned above. Not everything worked, but I was able to do it and I learned a lot in the process.
Debian --> Ubuntu (and its derivatives), Knoppix, Xandros, Lin(Free)spire, PCLinuxOS, and many more!
Slackware --> Vector, Zenwalk
That said, in my experience I have run derivatives more than the originals themselves. I have been installing Ubuntu for friends and family who are willing to try Linux as it seems so easy for them to use without the fun configuring that I enjoy so much. Their systems then are variants on a theme, while mine is in a world all its own - the benefit of learning configuring yourself. It takes a while, but you can tweak your system so it is unique, functional, productive, stable, and fun! Before you download a cd, research the community, how it's different from its parent distro, and don't waste time on distro's you won't use.
I have found myself settling on 2 distros of opposite backgrounds, that way I learn to use and configure more than just one type. I do not have much experience with rpm based distros (Red Hat) except for a short stint with opensuse so I cannot seem to be more help to people using them than for general issues. For Gentoo, Slackware, and Debian things I am continuing to become well rounded. That is my goal with distro's. I have picked 2 to use on my daily machines, and am continuing to use the others in different ways from rescue systems, to test boxes.
Conclusion - Download all distro's that look interesting to you, and try them out!
Happy tinkering :)