Yup, I use the following...
Linux (Redhat) on several firewalls running ipchains/iptables
Linux (Debian) on one of my laptops
AIX v4
SCO OpenServer r5

()
and of course all of my Windows boxen...
Win2k Pro on my primary laptop
WinNT 4 on several of our servers
Win2k Svr/Adv. Svr on several of our servers
etc.
Btw, for the most part, I use my *nix boxen entirely from the command-line. X? What's that?