MildMannered Industries has released Baseline 1.0, a disk sizing app for Mac OS X. It costs $10 to register; a demo is available.
Baseline scans your hard disk drive and lets you see how much each folder or file on the disk has grown or shrunk by and what its absolute size is, so you can see where your hard disk space has gone. Files can be deleted, compressed or archived within Baseline, and the software will warn users if they try to remove files and folders belonging to installed packages.
Baseline requires Mac OS X v10.4 or later, including Leopard.