Since I upgraded my MacBook to Lion (Apple's new version of OSX) I've noticed a few days ago I've been loosing hard drive space quite rapidly? After searching around the Apple community forums I noticed a few articles which pointed toward Time Machine backups, turns out Lion will backup using the local disk if it cannot find it's dedicated drive, a little bit of a pain. Since I'm a laptop user I find myself away from my external drives a fair bit.
There is a command you can use in the terminal to stop this from happening:
sudo tmutil disablelocal
This will completely stop Time Machine from using your local disk and return your drive usage back to normal. Running this command gave me 2GB of space back after a restart. It looks like this only happens on Apple portable computers.