A mind map can be a good way of getting a grip on legacy code (using the same flow as the article but to document legacy code and not a programming language). Freemind is excellent for this but I often just use a folding text editor. The main thing about either is that you can go from 10000ft to ground level and back up quickly which is what understanding a codebase is all about.
Something that I've used for this is Fitnesse. It is a wiki that allows you to document the legacy code and a test bench to execute tests against the code.
