Oldest one I still -own- is a SPARC Classic (the old lunchbox 50Mhz sun4m one). It sits on a shelf; I haven't used it or really thought about it for a while. I mainly still have it for nostalgia - its what I learned Solaris on back in the day (2.5 or 2.6 I think) when I was a lowly tech support drone at a small mom-and-pop ISP in the mid-90s, and I also learned a lot about UNIX/Linux networking by figuring out how to netboot slackware onto it. An ISP I sysadmined for from 2001-2006 or so used a stack of them for DNS and NTP servers. I also have a SUN Blade 100 that I retired as a desktop UNIX machine a few years ago as it no longer has the balls to keep up (Firefox/Thunderbird/etc bloat). It also got relegated to a closet as I can't bring myself to throw it away. I also had a few pizza-box Sparcstations and Ultrasparc/US2 systems as part of my personal lab over the years but eventually recycled them. None of the places I consult/contract for use Solaris anymore, so its hard to justify running multiple systems for testing when I can run a cheap dual-quad core Linux box with Xen VMs and simulate a large network on a single system.
The oldest machine I still have in use is an old Dell PIII (slot style!) that I run Windows 98 on to play some old games, like Jane's USAF, Dungeon Keeper/DK2 and Mechwarrior 3. It spends most of its time shut off, but every now and then I get a hankerin' for a spankerin'.