I second Scanner. I used to use some other program, but I've stuck with Scanner since Bitt first mentioned it here. I love that it's a portable app, too.

I know that WinDirStat will also do the job, but I just can't understand the love for that program, given that its output is just so damn hideous. I really don't understand the whole ugly gradient thing they have going on there. I can't imagine it would be harder to read if they just stuck with solid colors. It would still be ugly, but less so.