My 2009 MBP running 10.7.3 was running out of hard disk space. To the point of only 90MB available. Did all of the normal stuff - deleted Trash, looked for duplicates, etc. But in adding up all of the stuff in the directories, couldn't find where all of the space was being used.
Searched the forums and tried things like turning Time Machine off. Disabling local Time Machine Snap Shots. But still couldn't find out what was going on. I could select "Get Info" on the internal hard drive, and watch the space disappear without doing anything.
Finally figured out what was going on this evening. I had been looking in my home directory, and found that it had over 200GB of stuff, even though when I added up the sizes of each subfolder, I got nowhere close to 200GB. So I next made the hidden folders visible (Terminal command: chflags nohidden ~/Library/) and then looked further. The culprit was the Mail folder. And within that folder was a Yahoo recovered mail folder that had almost 200GB of stuff in it. Deleted that folder (also deleted the Yahoo account from Mail preferences which means I will need to access Yahoo Mail through the web instead of Mail).
That fixed it!
I hope this helps others that are loosing disk space for no apparent reason.