What's keeping that device in use?

Ever wonder what's keeping that device in use so you can't unmount it? It's happened to me a few times. The tool to discover this information? lsof.

Basically, you type something like: lsof /mnt/cdrom and it gives you a ps-style output detailing the PID and process of the processes that are using the cdrom. You can then go and manually stop those programs, or kill them yourself.