My main PC started it's life as a Zenith Data Systems 8088/8MHz... Over a span of years it's been upgraded piece by piece to a K6-2/400MHz. Obviously no parts remain from the original system, which was eventually reassembled and sold.
Other parts from the system have been recombined into systems themselves and upgraded here and there with spare parts. Some of my parts have been given to my mom for her li'l box.
I have a Micron Transport XPe laptop (Inhell 150MHz MMX, Win2k Pro, trying to find a linux distro I like for it) I use quite a bit.
I also have a Mac 7300/180 (with a 604e/233MHz upgrade) collecting dust.
And of course my Miggys... A4000D, A2000HD, two A1200 boards in the process of being repaired to build something with, and a friends A500 which he will probably never get back :-D
My operating system usage varies depending on what I'm doing at the time... I've used everything from DOS 1.1 up to Win2k... MANY distributions of Linux... BeOS 4 & 5... Various QNX versions... Even Slowaris, er, Solaris. I'd love to use AROS, but I'll wait till the native version becomes usable before attempting it again.
Needless to say, I'm fond of experimenting with my systems! :-D