Tools

ASTREE
A static analyzer analyzes reactive, critical, real-time programs written in the C programming language and looks for runtime errors.
http://www.astree.ens.fr/

C Module Viewer (CMV)
A small and smart programming tool that supports software engineering under C in the subjects modularity, overview and navigation (for Windows 9x/NT/2000). By Ezer Programmings.
http://www.ezer.de/

C to Java converter
Convert C preprocessed source to Java.
http://www.soften.ktu.lt/~stonis/c2java/index.html

CC Shell
Allows C source files to be directly executed as scripts (using #!), speeding development and simplifying maintenance.
http://ccsh.sourceforge.net/

CCured
Source-to-source translator for C, which analyzes the program to determine the smallest number of run-time checks that must be inserted in the program to prevent all memory safety violations.
http://hal.cs.berkeley.edu/ccured/

CodeSurfer
An analysis and inspection tool that lets software developers and maintainers easily understand detailed dependence relations in source code.
http://www.grammatech.com/products/codesurfer/

CodeViz
CodeViz provides the ability to generate call graphs to aid the task of understanding code. [Open Source, GPL]
http://www.csn.ul.ie/~mel/projects/codeviz/

The Codist
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.
http://members.tripod.com/codist/

Compilercache
Caches compiled object files to dramaticly speed up compiles. More convenient than make, and speeds up builds (like RPM builds) that make doesn't. Obsoleted by ccache.
http://www.erikyyy.de/compilercache/

Crystal C/C++ Suite
A suite of C/C++ review, visualization and editing tools: Flowcharts, Tokens Panel, Comment Generator, and Auto-Formatting in real time, call/caller/file graphs.
http://www.sgvsarc.com/

Cscope
Cscope is a developer's tool for browsing C programming code.
http://cscope.sourceforge.net/

Cxref
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.
http://www.gedanken.demon.co.uk/cxref/

Development Assistant for C
Supports a wide range of software development activities, from easy editing to controlling source quality, from detailed reviewing to documenting. By RistanCASE.
http://www.ristancase.com/

Exuberant Ctags
Generates an index (or tag) file of C language objects found in C source and header files that allows these items to be quickly and easily located by a text editor or other utility. [Open Source, GPL]
http://ctags.sourceforge.net/

GNU cflow
GNU cflow analyzes a collection of C source files and prints a graph, charting control flow within the program. [Open source, GPL]
http://www.gnu.org/software/cflow/

GNU GLOBAL
GNU GLOBAL is a source code tag system that works the same way across diverse environments. [Open source, GPL]
http://www.gnu.org/software/global/

GNU Source-highlight
Source-highlight, given a source file, produces a document with syntax highlighting. [Open source, GPL]
http://www.gnu.org/software/src-highlite/

Indent
A tool for formatting C code. [Open source, GPL]
http://www.gnu.org/software/indent/indent.html

Insure++
An error detection tool for C/C++ that automatically finds hidden errors at the source code level.
http://www.parasoft.com/jsp/products/home.jsp?product=Insure&/index.htm

KScope
KScope is a KDE front-end to Cscope. It provides a source-editing environment for large C projects, such as the Linux kernel. [Open source, GPL]
http://kscope.sourceforge.net/

LCLint
Tool for statically checking C programs. [Open source, GPL]
http://lclint.cs.virginia.edu/

Lint Plus
Source code analyzer, detects problems compilers cannot catch, by Cleanscape Software.
http://www.cleanscape.net/products/lintplus/

OpenGrok
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 Subversion. It is written in Java. [Open source]
http://www.opensolaris.org/os/project/opengrok/

Splint
Splint is a tool for statically checking C programs for security vulnerabilities and coding mistakes.
http://www.splint.org/