...is Linux Mint.
I've been running dual boot Windows XP/Linux systems on a couple of the computers at home for several years now and have tended to flit from distro to distro as the mood took me. Five years or so ago, SuSE seemed best to me - after a brief flirtation with MEPIS, I then got into Ubuntu only to be rather turned off by certain aspects of the user interface.
On some of the lower powered PCs at home I've been using Xubuntu for quite a while - basically Ubuntu running the lightweight xfce desktop. Fast and slim, but perhaps a bit skeletal for some users.
I read an article in Linux Format magazine about Mint that made it sound pretty good so I gave it a shot as a dual boot on my ageing Dell Latitude laptop. 2006 vintage, Intel Core Solo CPU, and running Win XP at an increasingly slow crawl. I'd uninstalled non-essential software, changed the anti-virus, checked for malware... all in vain. It takes about 5 minutes to go from power-on to a usable Windows desktop. On a good day.
So, on went Mint. Basically it's similar to Ubuntu but just slightly more user friendly. It looks a bit nicer too - not that that's in any way really important but it just tips the balance. A very well-designed green colour scheme. It recognised the wireless chip in the laptop immediately and configured it correctly, which has been a real bugbear of previous Linux laptop installations for me. Total time from boot-up to a functioning desktop? Less than a minute. No obvious bugs, package update and installation very smooth (as is Ubuntu's, to be fair)... everything in its right place.
I was so impressed with Mint I'm now using it as my Linux distribution of choice across my network. Great work, guys.