Linux Distributions: (attempting to be in order of first [real] use)
- Mandrake (10.1, later tried Mandriva 2008.0)
- Fedora Core (3)
- Knoppix
- SuSE
- Debian
- Ubuntu (6.06 - 8.10, Server, Kubuntu, Mythbuntu)
- Gentoo (2008.0 onwards -- current distro of choice)

BSD "Flavors":
- OpenBSD (obvious favorite; used as firewall for quite a while)
- FreeBSD (including FreeSBIE [liveCD version])
- NetBSD

Microsoft:
- Windows 3.1 (limited -- mostly simple games)
- MSDOS (also limited; too annoying for real use -- see Linux)
- Windows 98 (first prolonged introduction to Windows)
- Windows 2000 Professional (my personal favorite of all the versions)
- Windows XP Home/Pro (used for the longest period)
- Windows Vista Business (gave several good chances; Vista was a screaming failure)

Mac OS: (in order of release)
- 9 (very limited -- mostly simple games)
- X (10.3 limited, 10.4, 10.5 -- heavy use in both 10.4 and 10.5)

It should be noted that I now use strictly Linux, with Gentoo as my distribution of choice.
(This was something I'd been trying to do since before the first time I installed Mandrake right up until 2009 when I finally did it.)