TreeSize Free tells you where precious space has gone to. can be started from context menu of a folder or drive and shows you the size of this folder, including its subfolders.
Windows only: Open ZIP, ARJ, CAB, ISO and a wealth of other compressed files with ALZip. More capable than 7-Zip and much easier on the eyes (ALZip's interface employs cute, googly-eyed icons), ALZip can do all the usual compression-software tricks: split