Ubuntu

Ubuntu is one of the most popular Linux distro's. It owes it's success to Debian the distro upon which Ubuntu is based. Although Ubuntu is a new project, of only 2 years at the time of writing, the fact that it has risen to the top of the Distrowatch hit list, and stayed there, is testament to it's popularity. The main reason I believe for this popularity is their striving for ease of use and prettiness.

As a distro it use the Gnome desktop and hand picks 1 application to do each task, this makes it an ideal distro for new comers to linux. It has a new installer built into the live cd so you can try before you install and it only takes up 1 cd. It has an integrated and easy to use application installer at the bottom of the 'Applications' menu, which give a good general description of what you are about to install, if there is anything missing.

However it does have some draw backs, because it is based on Debian it has to abide by the free (as in speech ethos), this is actually a good thing, however it means you need to install any proprietary video/audio codecs, flash player etc. The good news is though that clever users of Ubuntu have put together an Unofficial installer which does all this for you.

Other Versions

Kubuntu This is Ubuntu but uses the KDE desktop instead of Gnome
Edubuntu This is Ubuntu maxed out with educational programs designed for schools and kids
Xubnutu This is Ubuntu with a lightweight desktop for older/slower machines