The most useful tool I have found is etags. With etags you can quickly jump to the source file where a function is defined. If the source file is not already open in a buffer Emacs will open it for you. First you need to generate a TAGS file. To create a TAGS file run

find . -name '*.[ch]' | xargs etags

in the top directory of the source tree. Now start Emacs. M-x visit-tags-table will prompt you for the location of the tags file. Emacs now knows the name and location of all function definitions in the source tree. To jump to a function use M-. (that’s Meta-Period) and type the name of the function. If you simply press enter Emacs will jump to the function declaration that matches the word under the cursor. Very nice. As useful as this feature is it would still be very annoying if you couldn’t easily move back to where you jumped from. To jump backwards use M-*.