I wrote my first plugin as a learning exerciese. There may be existing plugins that provide this functionality, but I wanted to do it anyway
The plugin allows you to set an include file search path, then open C/C++ #include files from source code that are under the cursor. It works for both library header files (#include <header.h>) and local header files (#include "header.h"). The default mappings are alt-d and ctrl-mousebutton3
You can find the plugin here:
Feedback is welcome. I hope somebody else finds it useful.