Boost C++ Libraries

...one of the most highly regarded and expertly designed C++ library projects in the world. — Herb Sutter and Andrei Alexandrescu, C++ Coding Standards

Other features

Default constructors don't allocate memory which improves performance and usually implies a no-throw guarantee (if predicate's or allocator's default constructor doesn't throw).
Small string optimization for basic_string, with an internal buffer of 11/23 bytes (32/64 bit systems) without increasing the usual sizeof of the string (3 words).
[multi]set/map containers are size optimized embedding the color bit of the red-black tree nodes in the parent pointer.
[multi]set/map containers use no recursive functions so stack problems are avoided.