The image in this post shows a tree where the interior nodes represent directories and the leaf nodes represent files in the PLT source code. The leaves are colored based on the programming language used. (To avoid clutter, if there is more than one file in a given directory written in a particular language, that language only gets a single dot.)
Some highlights: the blues are Scheme-like languages, the reds are langauges we use to write documentation (see Scribble for more about them), the greens are teaching languages, orange is the language we use to bootstrap new languages, and yellow is a language for metadata about nearby files.
Curious about how we managed to write and use so many different languages? I'll be giving a talk at Flourish 2010 next week (3/19 @11am, UIC in Chicago) explaining how. Come to learn more!