From their website, the description more or less tells the whole story:
When you start OmniDiskSweeper, it presents you with a list of disks attached to your machine. Double-click on one, and a new window opens with a “column” view listing every folder and file you can access, which it sorts by size as you watch.
You then simply browse through the folders and files and delete the large ones which you are no longer using. If a file is part of the system, it’ll say so on the panel (in the list of Packages the file belongs to), so you won’t accidentally get rid of something that would make your system stop working. The free space on the disk and the ordering of the folders are automatically recalculated. If you aren’t sure what’s in a file, you can open it by double-clicking on its icon or dragging its icon onto another application. It’s that easy!
While this concept may seem awfully close to what the Finder does, it’s actually amazing how much more useful it is for cleaning up your hard disks. Since the directories and files are sorted by size, you get to quickly zoom in on the big files that are taking up all the space and not worry about the thousands of tiny ones that aren’t hurting anyone.
Temporary files, videos you don’t want anymore, printer drivers for printers you don’t actually own, that 80′s music collection you probably shouldn’t be storing on your work machine … OmniDiskSweeper helps you find everything that’s hogging your drive and clear it out. That way you’ve got room for the stuff you actually need.
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