Disk Inventory X
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Full user review
"Perfect for people with full hard drives"
Shows exactly what you have at a glance, can sort by size so you see the biggest things first, allows you to open folders, and even delete things right from the graphical window.
A little slow, even after it has read every file (which is obviously limited by your disk speed and how many files you have). It has crashed a few times, but rarely and does not hurt anything when it does.
If you have a hard drive close to capacity and do not know why, this tool is essential. I found files I forgot were there taking up 10s of GB of space.
Much quicker and easier than (for instance) du or Finder's Get Info.