What tools and techniques do you use for exploring and learning an unknown code base?

I am thinking of tools like grep, ctags, unit-tests, functional test, class-diagram generators, call graphs, code metrics like sloccount and so on. I'd be interested in your experiences, the helpers you used or wrote yourself and the size of the codebase, with which you worked with.

I realize, that this is also a process (happening over time) and that learning can mean "can give a ten minute intro" to "can refactor and shrink this to 30% of the size". Let's leave that open for now.