For Unix, there's a tool called kdirstat, that uses treemaps, which are just a generally cool way of visualizing things like disk space usage.

You can get free TreeMap classes in Java or the original TreeMap stuff at U. Maryland. The original inspiration for TreeMaps was mapping out your directory usage, and it's built in to the Maryland tool (which slammed my hard drive for almost a minute, but managed to produce a decent map showing me that I'm burning an awful lot of disk space on my image files).