PC - Dell Dimension 2300 (I needed something in a hurry when I started university, and they could deliver) - normally I'd have put something together myself. Intel P4, 2.3GHz, 256Mb RAM, patheric i845GL onboard graphics, and I can't even remember what soundcard I stuffed in there, running Windows XP Home. It's a boring system, but is dead stable, and does what I ask of it without grumbling. Haven't even had the cover off other than to clean the cat fluff out of the air filters in over a year.
Also have the following:
Laptop - Zenith Data Systems Z-Note 1000. P166, 32Mb RAM, 2Gb HD, running Win98SE (slowly!). Bought for univerisity/college a couple of years back for about £80. Just a word processor I could take with me really, and it's served that purpose well. Now has started to develop graphics issues, and the battery's just about had it...but after surviving getting dragged back and forth to and from Aberdeen every day for three years and taking the beating it has done, I'm certainly not going to complain, especially given that it was ancient when I got it.
Also - in the "other" category. Amiga A1200, with a Blizzard 68030/50MHz accelerator, 64Mb of memory, 4Gb harddrive, running Amiga OS 3.0. This is all but retired now as I've got an emulator which is at long last stable, and runs just as quick on the PC, and I don't really have space in here for it as well!
Have got an Acorn Archimedes A5000 (Risc based machine) that I wish I had space to set up as well.
Really want to get a proper RiscPC at some point, but haven't managed to afford one yet. They seem to be holding their value astonishingly well!