Where can I find good open source code flow visualization software?

History flow's are very neat for changes/diff across multiple versions.

Codeplex has a project, Dependency Visualizer which does support C also.

Gprof2Dot can render oprofile, this would get you dynamic info also.

CodeViz also (static tool) would work.

If your using gcc, gcc-xml has an introspector plugin also todo this.

You appears to want to acquire a flowchart of C source code ("decisions", "code blocks").
Something like this C flowchart?

To do this correctly, esp. for Linux kernal code, I'd expect you to have to preprocess the code first to get rid of macros and conditionals. I would assume that GCC would construct such a graph internally and that you ought to be able to get your hands on that graph.

Perhaps a tool like KCacheGrind would be of help. It generates call graphs based on actual calls and cannot pre-generate a call graph without actually running the program, which may not suit your needs, but then it again it may.