Detecting superfluous #includes in C/C++?
Google's cppclean (links to: download, documentation) can find several categories of C++ problems, and it can now find superfluous #includes.
There's also a Clang-based tool, include-what-you-use, that can do this. include-what-you-use can even suggest forward declarations (so you don't have to #include so much) and optionally clean up your #includes for you.
Current versions of Eclipse CDT also have this functionality built in: going under the Source menu and clicking Organize Includes will alphabetize your #include's, add any headers that Eclipse thinks you're using without directly including them, and comments out any headers that it doesn't think you need. This feature isn't 100% reliable, however.
It's not automatic, but doxygen will produce dependency diagrams for #included files. You will have to go through them visually, but they can be very useful for getting a picture of what is using what.