Treemaps!! First heard about them at OSCON 05 when Tim O'Reilly mentioned them in a keynote.

The paragraph below is from Treemaps for space-constrained visualization of hierarchies

During 1990, in response to the common problem of a filled hard disk, I became obsessed with the idea of producing a compact visualization of directory tree structures. Since the 80 Megabyte hard disk in the HCIL was shared by 14 users it was difficult to determine how and where space was used. Finding large files that could be deleted, or even determining which users consumed the largest shares of disk space were difficult tasks.

Just today I found treemap on the CPAN.

Wonder how the Perl sourcecode could be treemapped? The linux kernel is.