WizTree

When the drive fills up, WizTree answers "where did it all go?" in seconds rather than minutes. On NTFS it reads the Master File Table straight off the disk instead of walking every folder, then shows a sortable tree and a treemap where the biggest blocks are the files worth deleting. Mind the license: it is free for personal use, but commercial use needs a paid supporter code. For an office PC where that is a problem, the open-source WinDirStat does the same job.