Ultrix (Yeah, I know, not Linux, but some things carried over!) ;)
OS/2 (Yeah, I know, not Linux, but a lot of free software was ported from *NIX systems into OS/2, so some of that carried over as well.)
AmigaDOS (Yeah, I know, not Linux, but it was *NIX-y)
Slackware
Mandrake (later Mandriva)
Red Hat 7.1
TurboLinux
SuSE
VALinux
VectorLinux
Knoppix
Morphix
Gentoo
Sabayon
RR64
Debian
Red Hat Enterprise Linux (much newer version, very different, worth mentioning again.)
rPath
CentOS
Fedora
Ubuntu
Overall, I think I learned the most about Linux from Gentoo, with CentOS being the number two.
(Other *NIX systems: Solaris, OpenBSD, FreeBSD and perhaps one might think about counting Mac OSX and BeOS)