Mac OS X only: Ever try ejecting a thumb drive or other volume on your Mac but your computer claims the "disk is in use and could not be ejected"? WhatsOpen hunts down the culprits so you can get on with your life.
Just download WhatsOpen and fire it up, choose the Volume you're trying to eject from the Volume drop-down, then click on the List Files button. WhatsOpen will scan that volume and detect and list any open files on said volume. Armed with that knowledge, you should be able to close whatever file or application is causing the problem and eject the disk. If you're still not having any luck, you can simply click the Force Eject button and be done with it.
Windows users looking for a similar solution should take a look at previously mentioned File & Folder Unlocker. WhatsOpen is freeware, Mac OS X only; donations are accepted.