Check this site for a survey of source code comprehension tools (these show flow and/or relationships of function within the program in a graphical and/or text manner). Both free and commercial products are listed.

OpenGrok is a fast and usable source code search and cross reference engine. It helps you search, cross-reference and navigate your source tree. It can understand various program file formats and version control histories like SCCS, RCS, CVS and Subve...

Source code analyzer, detects problems compilers cannot catch, by Cleanscape Software.

An error detection tool for C/C++ that automatically finds hidden errors at the source code level.

A tool for formatting C code. [Open source, GPL]

Cxref is a C source code cross referencing and documenting program for UNIX, written by Andrew M. Bishop. Cxref makes both html and latex style document.

CodeViz provides the ability to generate call graphs to aid the task of understanding code. [Open Source, GPL]

A static analyzer analyzes reactive, critical, real-time programs written in the C programming language and looks for runtime errors.

Splint is a tool for statically checking C programs for security vulnerabilities and coding mistakes.

Tool for statically checking C programs. [Open source, GPL]