Ground zero level: (below true hobbyist systems, for mainly businesses without geeks)
Linspire : A commercial distribution that uses an interesting packager "click 'n run." Considered insulting to the user.
Freespire : Free (beer) Linspire
Xandros: A commercial continuation of Corel Linux.
Excellent for beginners:
Ubuntu : The undisputed (I think) champion of this category. Very easy, friendly, and stylish (for those who care.) Unfortunately, it puts to many restraints on the user, such as no graphical access to root. The best GNOME support available, but it also comes in Kubuntu (with okay KDE support) and Xubuntu (XFCE based.) Uses APT (deb) package manager with either ADEPT or SYNAPTIC frontends. Has over 12,000 packages or so. Comes in a live CD form (only) with a simple installer.
User-friendly distributions that put less restraints, so they can be more powerful to advanced users if they please. I say it's the "perfect category."
Mandriva: My personal favorite. Is a very professionally done distribution with good KDE support and okay GNOME support. Uses a powerful RPM system, URPMI, and has an extremely powerful control center with RPMDRAKE, an URPMI frontend. Comes in live CD and other forms.
OpenSUSE : A popular distribution with YAST, a control center with large RPM repositories and other features. Has very good, even support for KDE and GNOME. Only comes with full installer CDs (or DVD,) except for a live DVD released after initial distribution releases. I find the live DVD to be poor quality. Unfortunately, I do not support this distribution, as its sponsor, Novell, has possibly broken the GPL and made a deal with Microsoft.
PC-BSD: The easiest BSD (not GNU/Linux) distribution. Has great KDE support, though it is rough around the edges, particularly when it comes to Linux compatibility. Remains internally strong, though. A one CD installer.
Mepis: My favorite Ubuntu derivative. It is fully compatible with Ubuntu, while having its own separate base (and partial kernel fork!) Amazing KDE quality. Comes in live CDs. Gives more power (like root access) than Ubuntu, but compatibility is an issue with certain computers. Perfect for hobbyists and enthusiasts.
PClinuxOS A live KDE fork of Mandrake (now Mandriva,) this great-distribution-with-a-bad-name now is testing PClinuxOS2007. It looks quite a bit like Mandriva now, but uses the SYNAPTIC frontend for its own RPM repositories.
KNOPPIX: A deb distribution that is mainly a live CD, but can be installed. Good as a repair toolkit, though I was unimpressed by the recent 5.1.1 release.
(Note: Damn Small Linux and Puppy Linux, micro-distros, also belong in this category.
Experience recommended (I like these, but I wouldn't recommend them to a beginner):
Fedora Core: This solid, free distribution is many peoples favorite, as it gives the user power. Unfortunately, this is no longer the favorite of Eric S. Raymond (Open Source Initiative leader.) It is powerful, though the YUM/RPM system is sketchy (see Raymond's rant.) Seems slightly more hostile then the distros in the last category, but remains easily usable if you know what you're doing. It has good community support. It installs from non-live CDs or a DVD (with the famous ANACONDA installer,) but will soon be installable from a live CD. One live CD without installation has been released. KDE works, though GNOME is the better desktop choice.
Sabayon: Using Gentoo's package manager, Portage (it works by compiling programs and resolving dependencies during the compilation) , this KDE distribution is a rising star (and rightfully.) It is still rough around the edges, but usable. It comes in a live DVD that contains everything (really!) It also comes in a live CD form. The DVD is slow, but has media centers and even Quake4 built in. It uses the ANACONDA installer for a fast, clean installation. You can even use it live during installation!
FreeBSD: As the base for the aforementioned PC-BSD, this is another true UNIX distribution, with similar traits. However, unlike its cousin, it is less oriented for the desktop, though is a great server choice. Can run GNOME nicely, though.
Open Solaris: Literally Unix (as GNU is Not Unix.) It works pretty well, and has many impressive features. The platform is strong and powerful, but the fact that it's Unix can be intimidating to a GNU/Linux person. OpenSolaris is not an operating system in-and-of-itself, though. I've used BeleniX. The live quality is impressive, and contains many good programs, though lacks Linux compatibility that can run some programs that one might want. Not friendly to anyone that really isn't a Unix person, though usable.
More advanced distributions like Debian, Slackware, and Gentoo, are really not for anybody without plenty of experience. I consider those the best distros for people who are not experts. My opinions are only my own, and do not represent yours or any OpenSUSE user who does not want to be damned to hell by a GNU/Linux obsessed atheist;)