Mainstream Distro'sUbuntu Desktop,
Server,
Netbook RemixList of official mirrors (inc ISP ones)
ClickyVery easy to use, aimed at beginners but with plenty of room for growth. Has a rigid 6 monthly release cycle - April and October every year. A fork of Debian that uses the APT package manager. A touch of megalomania has started to set in lately. Negative in the freedom dimension, as Richard Stallman would say.
Debian hereLike an uncut version of Ubuntu, this is what Ubuntu is based on. Uses unaltered versions of Gnome and has many of the familar applications under a different name - firefox is called Iceweasel, for instance. For more advanced users.
Suse hereFor more experienced users, uses the YAST package manager. Backed by Novell.
Fedora hereA testing ground for the paid version of Red Hat and a die-hard open source distribution. It's commitment to open source means that media files probably won't work out of the box, but compatibility can be added later. Another for more experienced users. Uses the Redhat Package Manager (RPM)
Gentoo hereMost definitely a hardcore linux distro for people who speak in machine code and know how to "bash" their way out of a sticky situation. The upside of all this is the performance. Gentoo uses the "Portage" package manager which compiles each application for the user's machine. This means it could take hours to install a program from the repository, but at least all the scrolling text will look cool to onlookers! It also means that the program will work great (in theory). Gentoo also gives the option of using the linux kernel or a FreeBSD one.
Arch hereAnother seat-of-the-pants distro for people who like to live on the bleeding edge. Unless you really know what you are doing, you might want to try Manjaro.
Forks and variants...Manjaro hereBuilt on the Arch repositories, but with much of the scary stuff taken care of. Bleeding-edge packages in a user friendly way. Also a rolling release, so no periodic upgrades or reinstalls.
Ubuntu Studio hereA media-focussed version of Ubuntu, but unlike normal ubuntu, Studio features a "realtime kernel" which lends itself to audio and video recording but not necessarily normal desktop uses, so only for the die-hard audio & video production types! It comes with it's own fancy themes and wallpapers, which can be obtained from within a normal ubuntu installation by typing
Linux Mint ClickyAnother Ubuntu/Debian fork, priding itself on its user-friendliness and the presence of all of the media codecs you need out of the box. Definitely worth a look if you are just getting started, but be warned that there's no upgrade process. You'll have to reinstall every 6 months unless you go for a LTS release.
Kubuntu hereThis is just Ubuntu but using KDE. Resource usage is higher and it tends to be a bit buggier than Ubuntu, but it is certainly pretty. For Ubuntu users, you can have the best of both worlds by typing
from within the Ubuntu command line. You can choose between them at the login prompt.
Xubuntu hereLike Kubuntu, this is just Ubuntu but using the XFCE desktop. It is ideal for those with older machines thanks to the modest system requirements.
Sabayon hereUser friendly Gentoo based desktop distro. Always cutting edge - sometimes too much so.
PC Linux OS hereMandriva based destop distribution - user friendly, very stable, adaptable, easy to keep up to date via rolling updates (good for those who don't like the 6 monthly release cycle of most popular distros).
CentOS hereLegendary server distro, recently dragged into the Redhat fold. The CentOS project takes the Red Hat Enterprise Linux source code and packages it up for people to use. It's essentially RHEL without the support.
Free Unix distributionsThese are in many ways similar to the conventional linux distributions, but with a different underlying core.PC-BSD hereWith FreeBSD underneath and KDE4.2 at the surface, this is an attractive and easy to use distribution that regrettably has no Nvidia (or ATI as far as I know) GPU driver support for the x64 version.
OpenSolaris ClickyFrom Sun Microsystems (now Oracle), Opensolaris is the beneficiary of the work done on the commercial Solaris platform. Using the ZFS file system and the Gnome desktop, this is probably the one to go for if you want a Unix distro for free.
Debian BSD hereDebian userland on top, FreeBSD underneath.