I'd suggest the majority of the standard library is underused.
I'd further suggest that good portions of the standard library is of little use to games development.

/edit:

For underuse examples, most people will write their own simple functions rather than relying on the algorithm library. Some of the ordering and searching functions are used occasionally, but few people will use std::transform(), std::generate() when they need to come up with a bunch of items. I've seen programmers run full sorts rather than functions like stable_partition() or partial_sort() when they need just a few items. And almost nobody normally uses the heap functionality, even though it can be a very solid alternative to sorted containers, especially when it comes to bulk changes.

For not applicable functionality examples, the entire Localization library of the C++ standard library is difficult to apply to game development; it doesn't play nice with the major game development localization tools. The Diagnostics library is of limited use -- most games implement their own custom asserts, warnings, and such, and many game companies have exceptions disabled for better or worse. RNG in the standard library is not sufficient for games. Most of the IO libraries are useless for games because they are blocking. Etc.

Yeah I first learned about member function pointers when I started using wxWidgets.
operator() is pretty cool too.

That you can read and write from different elements in a union if the elements are structs and have the same prefix structure:


union {
struct {int x, y; };
struct {int u, v; };
}

x = 45;
cout << u;


I also like that the library and language is designed in such a way that the compilers can optimize usage of the library really well; gcc can optimize through almost everything in there.

I personally love quite a bit of the newish C++11 functionality, which is still not used by a lot of C++ developers I know, simply because they do not know of the existence of said features and because not all the features are supported by all the compilers.
Especially std::function, auto built-in smart-pointers are some of my new friends

I think a lot of features are just not known to many developers and the STL contains a huge load of stuff that comes in very handy quite often, but one has to know that it is there in the first place

RNG in the standard library is not sufficient for games.

Just FYI, there is a better RNG in the standard library for C++11 (Also higher-accuracy cross-platform timers).
C++11, while mostly focused on improving the core language (which it did), does add a few things of interest to the standard library that will help game developers moving forward (such as easier multithreading and the TR1-promoted smart pointers).

I'm personally trying to learn the algorithms that have always been in C++, since I've not much experience in them; later I'd love to learn the proper usage of streams.