I found this Wikipedia entry on the C standard library which contains, lists of C header files and detailed information on which standard they're part of. That gives you a nice historical perspective and some other, similar details.
Of course that's just C. There's a similar article to be found under "C++ standard library". That also has references to some other libraries which may not be "standard" per se but without which C++ would feel "crippled" to some people used to working with the extensions.
