WinMerge = http://winmerge.sourceforge.net/
ExamDiff = http://www.prestosoft.com/ps.asp?page=edp_examdiff
Exam Diff Page @ Download.com
I'm not up to speed on C programming tools, but both of these utilities do an excellent job of displaying changes of code.
Exam Diff also has a binary compare capability, I don't think WinMerge does and both can compare entire directories.
Useful for comparing changes from one source to another.
(Maybe this is what CVS does but I'm not familiar with that).