Source Code Browsers?

patonw asks: "I just started working for a company as a programmer on a project with a huge existing codebase. The person hiring me half-jokingly said that it usually takes new employees two years before they understand the system. What I am looking for is not just an editor/browser but a program that displays functions and classes as connected graphs -- preferably free. I would like to view how programs are structured by function calls and class relations. I have access to several different kinds of platforms/operating systems."