I used to use SequoiaView, but this application is better. It scans your drive(s) and builds a tree structure that lets you easily drill down to figure out what's eating your disk space. It also generates a treemap (I understand the term, but there's gotta be a better name- the visual looks nothing like a branched structure) that looks almost identical to SequoiaView.
The bottom line: This Windows application does one thing, show you what's taking up space on your drives, and does it very well.