What should I do with some extra old hard drives?

I was digging through my shelves earlier and found a bunch of old hard drives. I’ve recently upgraded to 1 and 2 TB drives on my desktop, but I never threw the old ones away.

I have a 320gb, a couple of 640 gb, and one other. They all seem to work just fine (I spun them up) and saw what was on them (nothing).

I hate to throw them away, but then, what do I do with a bunch of hard drives that are just sitting around?