My old kit…
Sun GDM-5410 20" CRT which runs happily at 2048x1576 resolution. This is the monitor I use on my desktop (AMD Phenom IIx6)
Silicon Graphics Indy (R4600SC CPU at 133MHz, 256MB RAM). Still works, but the PROM battery has died, so one has to dig up the MAC address and punch it back into the PROM if it hasn't been turned on in a while. It has Gentoo Linux/MIPS on it.
I also have the Indy Presenter 15" LCD, with removable backlight (for placement on an overhead projector), and a SGI 20" monitor to go with it.
Silicon Graphics Indigo2 (R10000 CPU at 195MHz, 384MB RAM). No longer goes, but makes a good door stop. The hard drives also have Gentoo Linux installed. Fun and games, because the Indigo2 was never designed for a CPU like the R10000, so gcc and the kernel need hacks to work around the hardware bugs that arise.
Silicon Graphics O2 (RM5200 CPU at 300MHz, 128MB RAM). Still goes, I upgraded it from a R5000 CPU, then had fun and games updating the PROM to support it. It runs Gentoo Linux as well.
Silicon Graphics Octane (R10000 175MHz, 128MB RAM). No longer goes, I think I killed it trying to clean out the dust. One SCSI port was always bad, as the PROM used to sit there for 10 minutes trying to initialise it, then the machine would come good. I had it running Gentoo, which was fun because the power supply serial number wasn't recognised, so I had to go patch the kernel if I wanted the keyboard to work (don't ask). Being a 175MHz unit, I'm guessing this must've been one of the first Octanes released.
SGI Indy, Indigo2 info
SGI O2 info
SGI Octane info
Gateway Microserver (rebadged Cobalt Qube II; RM5231 at 250MHz, 128MB RAM). Still goes, although the hard drive is dead now (I have some spares). It too ran Gentoo, in fact was the main build host for the Cobalt stage3's during my time working on Gentoo. The RM5231, which lacked secondary cache, was agonisingly slow at times. The boot ROM doesn't like bigger hard drives (I tried a 160GB and it failed miserably).
Whilst nowhere near as old, I also have a Lemote Yeeloong netbook PC. I bought this direct from China during my time at university, and was a great little machine. Sadly, the original power supply died, and the replacement was nowhere near as good, so it didn't get much use after that. The battery has also died. It still goes, runs Gentoo Linux/MIPS.