Fellow DfwPPerAdamKeysblogged about the TreeMap today. As is not too unusual, I'd never run across these before even though they have a long history (relative to technical time scopes).
One common technical use for the TreeMap is for analyzing disk space usage. As I have built my own tool for navigating through the depths of a hard drive, I really enjoyed checking out SequoiaView, a free TreeMap tool for analyzing Windows disk space usage. Download.