I recently noticed that the available disk space on my boot disk had diminished considerably. I couldn't imagine what had eaten over 7 GB of disk space, so I used OmniDiskSweeper to find the culprit. It turned out to be my mounted iDisk which had increased in size to 8.2 GB after Apple updated the storage capacity.

If you have an always mounted iDisk and now have lost too much needed hard drive space, you can unmount the disk and regain the space. Open System Preferences and click the .Mac button. Next, select the iDisk tab and click the Stop button under "iDisk Syncing On." Your iDisk will be removed from your Desktop and hard disk and you'll regain all that lost disk space.