In short: GrandPerspective is a file visualiser. It scans your hard drive for you (or certain folders within it), and graphically shows you what's taking up so much space. Like this!
Pictured is my work laptop. Each box is an app or file — it'll tell you what it is when you move your mouse over it. It's all vividly-coloured to help you distinguish between things, and you can zoom in if you need to. My huge email archive takes up the most space by far.
In contrast, here's my personal MacBook.
The email archive is the same size, but the hard drive is bigger overall — so the emails pale in comparison next to my music collection.
next slide will load in 15 secondsSkip AdSkip AdHere's one more — one of my colleague's personal laptops.
Here's what some of that is.