Older custom built computer, but not by me, 450Mhz AMD k6-2 with... err, 256MB of RAM, I think ("upgraded" from 512, since 256 is the maximum cacheable size). Still works, last I checked. I only have one monitor, though.
Dell Dimension 4400
I was going to set this up as an SVN server until I realized that was sort of a dumb idea, as I could easily set up such a server on either my laptop or my primary desktop machine. I believe it currently has an older version of Linux mint installed (9, I believe); Again, I have only one monitor so I don't often run this.
IBM Thinkpad 755CDV (two of them) neither one works, anymore. They did serve me well; well, one of them did. The second one was supposed to work so I could just take the drive from the busted one and put it into the "new" one, but apparently it doesn't function either; for 20 bucks I'm not surprised. It did have double the RAM of mine (16MB as opposed to 8MB). I voted for "lenovo" for these, even though they were quite a while before IBM sold off the laptop division to them.
Toshiba Satellite Pro 440CDX. Still works; Windows 98SE, 144MB of RAM, crappy passive matrix screen that's nearly impossible to read (I'm surprised I used to play quake on it, to be honest; terrible screen and awful framerates... No network connection, which is fine, since it has no reason to connect to anything. Doesn't really have any real use for me now that I have a more powerful laptop...
Toshiba Satellite L300, a nice upgrade to the older one, even if it isn't "pro"... Running Mint 10 and Win 7 in a dual boot configuration, 4GB of memory, some kind of Intel Dual Core at 2Ghz whose model number I haven't bothered to memorize. Mostly use it for watching videos/DVDs, and of course as a backup for...
My current main desktop- shown in my specs.