Linux
From Walter
I've been a Linux user since early 2000. I consider Slackware, Debian, RedHat (now Fedora), SuSE (now openSUSE) and Mandrake (now Mandriva) to be the only real distro's. Yes, I'm not an Ubuntu fanboi, and I do prefer rpm and KDE.
I currently use Fedora 10 on the desktop and CentOS 5.3 on the server. At work, I am experiencing a steep learning curve with z/Linux...
My favourite distro is a small home-brewed, backyard distro by a very eccentric Jay L. Klepacs (a.k.a. linuxkid). It used to be called Peanut linux and has since been renamed to aLinux. The distro is rpm-based and KDE-centric.
Google search distro trends for the top distro's.

