Lubuntu: One Honkin Fast Lean Mean Distro
You know, when you look in the Linux parts bin, you see bolt-on guis like KDE, Unity (only Ubuntu), Xfce, LXDE, Enlightenment, Gnome3. And you see a division of packaged Distributions along Gentoo's Emerge, Arch Linux Pacman, Red Hat RPM, and Debian deb package systems (those are the major players). Everything else at Distrowatch is combinations and permutations of the aforementioned parts with different philosophies regarding File Hierarchical Structure, and release management strategies combined with various community governance models.
That's fine and once you've sorted that out, you'll realize there is a lot of chaff. To varying degrees, you'll find some Distros are good and some less so and only a few with real good support behind them.
I've told you about which Distros will be around in five years and so the base Distros are most likely to be left standing while the others will have end of life'd officially or simply will lose support by attrition and die off due to lack of maintenance.
The other day I switched from Netrunner Enigma to Lubuntu 13.04. Let me tell you, my opinion of Lubuntu has risen significantly since my last experience a few years ago. Lubuntu is an Ubuntu derivative, of course, but based on the LXDE desktop gui.