I have a 1998-vintage Performa 6500/250 that still gets occasional use to transfer data off Mac-format floppies; also has Ethernet and USB cards added (previously had an upgraded ATI GPU in place of the USB card).
We also have a couple of Yikes G4 towers running 10.4 that actually get used regularly by students at work. That's a little sad.
These don't really count, as they're not getting used, but as far as machines that still work I have an original LC, ca 1990 (16MHz 68K chip, upgraded to 12MB RAM of which it could only use 10 if memory serves), which was my earliest Mac (I can't remember if it's that LC or another, but there is an LC with the Apple II personality card, which is cool), and an Apple //c and //c+, plus a Centris 610 and Quadra 6100AV (I think; can't remember the exact model number, but it's the same form factor as the Centris, which I always kind of liked).