I have to put in my usual recommendation for Ubuntu.
It is based on Debian and uses a Debian package management system, which by default uses Ubuntu-specific repositories and, even though I recommend adding other Ubuntu repositories such as the Multiverse and Universe becuase you can't do anything without them, I highly recommmend against using Debian repositories.
The great thing about Ubuntu is it's very problem-free in setup, so it's simple to install for most 'newbies', yet is a capable and advanced (and NOT bloated *cough*SuSE, Mandrake, RH!*cough*)...I keep recommending this becuase of how much I like it, and I've had some sort of version of every distro recommended here (and more) with the exception of Gentoo, which isn't exactly as 'newbie friendly'...particularly in the setup.
Yes I'm biased.