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Slackware is, and most of its derivatives like SalixOS. The mainline comes with older KDE version: 4.8.5 -- but a longtime contributor, Alienbob, provides recent compilations at http://alien.slackbook.org/blog/kde-releases-4-9-3/

There you go: http://goo.gl/ZowwQ

64 GNU/Linux distributions that use KDE. Rock and Roll!

I personally use ArchLinux x86_64 with KDE 4.9.3 => UberAwesome!

OpenSuse seems like a good one.It uses KDE by default.

This is technically incorrect, since both Gnome and KDE are first-class citizens (and so is XFCE). You get to choose upon installation, and all three DEs are in the official main repository.

(This is just a minor correction. By all means, go with openSUSE, KDE support has been excellent for me over the years.)

I have only heard good things about Chakra[1], but unfortunately I have yet to try it out myself. It's based on Arch linux and tries to be as pure KDE/Qt as possible.

[1] http://www.chakra-project.org/

OpenSUSE hands down.