Personally, I prefer just seeing raw text on a screen, without a fancy IDE, just a plain vi session. If it is my own code base, then I already know what is where, and can navigate / update very efficiently. If however I am working on someone else's code base, then it is necessary to study it for some time before diving in. What can really help in this case is a "code map" -- a document that gives a tutorial introduction and reference to the code, what is where, and why, etc. Included in the code map would be both program flow, and data flow (what data goes in, how it is transformed, and where it ends up). So what is needed is tools to help maintain that code map document so it doesn't get out of date.
