MacSlack calculates the disk space you would save by moving up to HFS

MacSlack is a freeware drag-and-drop application that calculates how much slack space is present in a given set of files. It can also predict how much hard drive space you will regain if you split your hard disk into more partitions or convert volumes in the standard format (HFS) to the new, more efficient format (HFS ) introduced in Mac OS 8.1. (Thanks, Jack Lau .)