Talking about .hpp extension, I find it useful when people are supposed to know that this header file contains C++ an not C, like using namespaces or template etc, by the moment they see the files, so they won't try to feed it to a C compiler!
And I also like to name header files which contain not only declarations but implementations as well, as .hpp files. like header files including template classes. Although that's just my opinion and of course it's not supposed to be right! :)
