Microsoft:
Windows 3
Windows 95
Windows 98
Windows 2000
Windows ME
Windows XP
Windows Vista
Windows Server 2000
Windows Server 2003
Windows Server 2008
Apple:
Mac OS 7
Mac OS 8
Mac OS 9
Mac OS X
Linux:
Ubuntu 6.04
Ubuntu 6.10
Ubuntu 7.04
Ubuntu 7.10
Ubuntu 8.04
Ubuntu 8.10
Several Ubuntu Server versions.
Gentoo
Fedora
OpenSUSE
BSD:
FreeBSD
So that would make either 3 or 23 (without the linux servers).
DOS
Windows 3.1
Windows 95
Windows 98
Windows 98 SE
Windows ME
Windows NT
Windows 2000
Windows XP
Windows Vista
Windows 2000 Server
Windows 2003 Server
Windows Home Server
Mac OSX
Ubuntu Linux
I have tried: Linux: Ubuntu, Suse, Redhat Fedora, Mandriva, Knoppix, and for a short time OpenBSD. Windows: Win95, Win98, Win2k, WinXP Mac: Tiger and Leopard