The Boost libraries are a collection of open-source libraries that extend the functionality of C++. Many of the Boost founders are active in the C++ standards committee and as a result several of the Boost libraries have been included in the new C++ ISO standard (C++0x), including smart pointers, function object binders, type traits, and tuples. You’d have to go a long way before you find better examples of modern C++ API design.
You can also find a good overview of the Boost libraries on Wikipedia.
