Well, we've got a research instrument (a spectrometer) running off an old 66 MHz 486 I donated to the lab; we use it all the time, so that is one still used and often used SLOW machine.
Unfortunately, the instrument is designed to be run off DOS software written for a computer based on a 486 chip, so we CAN'T upgrade. In fact, I brought in my old 486, gathering cobwebs in my basement, because the original 100 MHz 486 DX4 croaked.
Man... I tell you I used to lust after the DX4, but after this more or less direct comparative observation, there really isn't much of a speed difference between the two!