I still use Knoppix as a crash/infection rescue disk for family, friends, and clients who do dumb things.
On this particular workstation, I've got hard drive cartridges to swap OS flavors.
I've worked with Knoppix, Debian, Mandrake/Mandriva, Ubuntu, Fedora, PC Linux, FermiLinux, Debian, and SUSE.
SUSE, for now, is my favorite.
However, I'm posting this via a Windows 7 Beta installation on yet another hard drive cartridge.
Otherwise, I normally revert back to WinXP Pro SP3. It's really the least hassle.