Knoppix and the SuSE live eval cd are ways to see linux from a distance. They are run off of cdrom so no tough installation is necessary. Of course, you do not get a real feel for the system, but if you stilll arent sure you really want to try, this is probably the way to go.
Choices like RedHat, Mandrake, and SuSE are generally what you hear given to newbies looking to dip their toes in the water. These three companies are breaking new ground in usability on the linux front. They each have utilities to help install software and maintain older software. Their gui tools are typically installed on only their systems, so moving to another distribution after using one of these may be sort of a culture shock, especially if you choose one of the other distros I mention. These distros also generally go with bleeding edge, or damn close to it, software. They are also known to modify the default kernels a bit, but its typically not a problem. If you want to use it without necessarily learning much, this is the way to go.
Debian, Gentoo, and Slackware are a different breed:
Slackware wants to be the most "unix-like" and may be tough for some newbies. I personally liked Slackware 7 better than the current RedHat (at the time) because it was tough. I got to do things by hand instead of letting a system that may fail handle them for me. It taught me a lot by forcing me to do things.
Debian is for the GPL nut (yes, I say that just to annoy Nothinman ). Debian has a confusing install, if you dont read the instructions. It forces you, like Slackware, to do the work yourself. However, unlike Slackware, Debian has one of the most advancned software installation systems out there, for *any* OS. Using simple front-ends you can select from thousands of software packages, have the software download that package and any dependencies it may have, and install them for you. Typically this is a pain-free procedure. It can also update the base system and all packagess you have installed.
I will take a break for a second before going on to Gentoo to make a quick note abotu both Debian and Slackware. Both of these ditributions are fairly serious. If you want to learn the nitty gritty, this is the way to go. A nice mix of pre-built system, with dependancy on knwoledge. Both distros are a bit behind, but security fixes have been back-ported. They are very stable (even their unstable branches are pretty much production ready).
And on to Gentoo. I have not used this distro yet, but I have heard good things about it. It requires software to be compiled locally (and most of the system from what Im told) using their "emerge" system (similar to BSD ports). It is supposed to be fast (unless you are waiting for software to compile ) and fairly stable. They do however, have some fairly bleeding edge software. It sounds like a mix between the Debian/Slack camp and RH/Mandrake/SuSE camp.