How can I visualize the file system usage on Windows?

"TreeSize is pretty sweet.

Its advantage over the others is that on NTFS drives, it works on the MFT (Master File Table) and reaches extremely high scan speeds.

They have three versions of the product: Free, Personal and Professional.

The free version doesn't have fancy visualizations and reports but should be sufficient in most cases."

I tend to stick with Scanner. I occasionally try programs with more features, but I find Scanner does everything I actually need.

"I like FolderSize, since it's integrated into Explorer and caches the results.
You always know how much space a folder is taking, and can easily identify space hogs. It's always there, so you don't have to start a separate program."

"It's old, but I really like the tree map view in SequoiaView.

SequoiaView is free and happily works all the way up to Windows 7 and Server 2008."