Following Nikos and Vagelis here's a (possibly incomplete) timeline of my personal OS usage:

• MS-DOS (~1990) , on a 486dx-based tower (even had "turbo"!), mostly for games
• System 6 (early '90s) , on a Classic, for games, paint and word processing
• Red Hat 6 (1999), brought over by a friend to try Linux for the first time.
• SUSE Linux (2000?), flirting with graphical configuration tools like YaST. Bought an original CD along with an original copy of CodeWarrior IDE! This was the first time I bought original software. Previously I had only used "friendly" copies (either for free or through the shops around Stournara street). SUSE was the last Linux I have used on Desktops or workstations.
• Tried NetBSD (2002) but after a while I tried OpenBSD and ended up ordering a copy of OpenBSD 3.2. Still have that home firewall box lying around somewhere. Used OpenBSD until 2008.
• In the era of unsupported laptop hardware (2003+) I have used the following Linux distros (in no special order):
  • Debian GNU/Linux (Starting with "woody", currently running "jessie" on a netbook)
  • Ubuntu 5 Hoary Hedgehog
  • Arch Linux
  • Slackware
  • Funtoo (Rolling-release, one installation on a laptop at the time of this writing)
• In 2008 most colleagues ran Debian GNU/Linux so I attempted a compromise with Debian GNU/kFreeBSD. Used it as a personal workstation at work for about a year, great project! However the effort required in configuring, tweaking and bug reporting eventually got the best of me so I switched to...
• FreeBSD! (2009-currently) The features in CURRENT at the time (VIMAGE, DTrace, ZFS with GPT) were just too great to ignore (especially ZFS not being experimental anymore). Currently run FreeBSD 9 on a workstation and on a Thinkpad X series laptop. If  my other hardware wasn't broken or unsupported in some way I would run it everywhere.