Having a drawer built into your computer can be a great way to use an empty 5.25" bay slot. If you have an old 5.25" bay drive -- like a CD or DVD drive -- you can make your own drawer and disguise it.
A hidden drawer would be a great place to stash flash drives or discs with information you'd rather keep to yourself, and this guide from Instructables user tomatoskins can show you how to do it.
You'll need an empty bay in your computer tower (including the bay cover), an old 5.25" drive of some kind, a push-open push-close latch, and a few other minor components. When it's all set up, you'll be able to push the seemingly empty bay slot and the drawer will pop open. It's possible to buy drawers like this online, but when you make it yourself, it's a lot easier to disguise it. Check out the complete guide at the link below.
Hidden Computer Drawer [Instructables]