Download of MacSlack 1.0 for Mac was on the developer's website when we last checked. We cannot confirm if there is a free download of this app available. The program lies within System Tools, more precisely System Optimization. Please check the Mac app with an antivirus before launch as it is downloaded from the developer's website, and we cannot ensure that it is safe.
From the developer:
MacSlack is a drag-and-drop application that calculates how much slack space is present in a given set of files. It can alsopredict how much hard drive space you will regain if you split your hard disk into more partitions or convert volumes in the standardformat (HFS) to the new, more efficient format (HFS+) introduced in MacOS 8.1.