I love operating systems.
I use Windows and Mac OS X daily and like them both for different reasons. I can't wait for Tiger to arrive.
I try Linux every few years ever since Slackware 2.0 landed on my desk courtesy of a computer magazine's cover CD in 1994. I still find it too inconsistent for my daily use but appreciate the engineering behind it, the competition and mostly the educational benefits of having a full open system available for inspection.
If I had to run a *nix I'd probably choose a BSD flavour.
I used BeOS from r3 Intel up to r5 Dino beta on my main system and before that OS/2 2.1 to Warp for well over a year while I ran a multiline BBS.
Before the joys of OS/2 I used DesqView on top of MS-DOS.
Long before that I enjoyed Acorn DFS, Sinclair's BASIC and CP/M 3.0.
I've experimented with OpenStep, OpenServer and QNX at home. At work I've had the opportunity to use NetWare, AIX and IBM's OS/400.