C: Diagramming A Large Library

Doxygen can generate various types of interaction diagrams - files, function calls, etc. With the EXTRACT_ALL configuration variable set, it will generate diagrams and cross-referenced documentation even for code without documentation comments.

CppDepend, if you work on Windows, extends Doxygen with a query language and ability to move in the dependency graph.

Doxygen as mentioned by Nick Meyer is probably a good bet. You can also try the classic cross-referencing tools ctags and cscope, that you can use as standalone tools or integrated with your text editor.