Distributions I've used (or at least experimented with):
* = Run in a production environment

Arch
DamnSmall
Debian
CentOS*
Fedora
Gentoo*
gOS
Mandrake
Mepis
Puppy
Redhat (pre-Fedora)*
RHEL*
Sabayon
Slackware*
Suse
Ubuntu*
Vector
Zenwalk