Carol Hart has an extra hard drive hanging around. She asked me what she can do with it.
Old hard drives make lousy flowerpots, but very effective paperweights. And I must confess that on some occasions, I've been tempted to use them for batting practice or skeet shooting.
But I don't think that's the answer you're looking for.
One thing you can do with an old drive is give it away. Check around to see if there's a friend, local school, or non-profit that could make use of it.
Before you donate the drive, however, you need to make sure that there's nothing of a private nature on it. If there are files that might embarrass you, or that might prove valuable to identity thieves, you need to properly destroy them. See How to securely wipe sensitive files--or your entire hard drive for details.
You can also make use of the drive yourself. If your computer has an extra drive bay (very likely if you have a desktop; very unlikely if you have a laptop), you can install an old internal drive and gain a bit more storage. Assuming that the drive connects via today's standard SATA interface, this is a very simple operation. Older drives may use an IDE interface, which your PC's motherboard might not support.
Finally, you can turn an internal drive into an external one. All you need is a USB enclosure. These generally cost about $20, but I've seen them for as little as $10.
One more thing: If your PC doesn't recognize the drive, see My PC doesn't see the new, second hard drive. And while that article was about internal drives, the relevant advice works for externals, as well.