Here we will make an attempt at describing the non-Standard extensions to the library. Some of these are from older versions of standard library components, namely SGI's STL, and some of these are GNU's.
Before you leap in and use any of these extensions, be aware of two things:
Non-Standard means exactly that.
The behavior, and the very existence, of these extensions may change with little or no warning. (Ideally, the really good ones will appear in the next revision of C++.) Also, other platforms, other compilers, other versions of g++ or libstdc++ may not recognize these names, or treat them differently, or...
You should know how to access these headers properly.