Lots of motors, solenoids, and sensors in these. And of course, the electronics to control them, (if you don't mind desoldering them from PCBs), and the power supplies to power them! Also plenty of wire, LEDs (though they are dirt cheap nowdays), microswitches, bearings, springs, etc. etc.
I'm talking about the bigger floor models, not a little plastic desktop model.
The "finishers" (the parts that staple, hole punch, etc.) are especially great because they usually have a high proportion of motors and sensors.
Companies will sometimes hold on to old copiers because they don't get any trade-in for them, but a copier that's >8 yrs old is worth virtually nothing on the wholesale market and dealers don't want to service them. So companies that hold on to old ones find that they just sit in storage collecting dust. Eventually they realize, "yeah I guess it really was worthless" but by then they also discover that they are going to have to pay to get rid of them.
So if you come along at the right time, you can probably pick one up for free.
Even more likely....go to a local copier dealership and you will very likely find they have a machine (or more than one) that is waiting to go in the dumpster that they'd be more than happy to just give to you!