Ever forget exactly which file you had placed some particular code in and really wanted to find it quick. Here's a grep oneliner to do just that. In this example, I am looking for "#define MAX_VALUE" in a directory containing many source files and sub-directories.
grep -R --include "*.c" "#define MAX_VALUE" .
Note: the "." indicates the current directory in Linux. If your files are in a different tree, just replace the "." with that tree's root location. For example, if you wanted to look /usr/bin/local/:
grep -R --include "*.c" "#define MAX_VALUE" /usr/bin/local