Boost provides free peer-reviewed portable C++ source libraries.

Boost emphasizes libraries that work well with the C++ Standard Library.
Boost libraries are intended to be widely useful, and usable across a
broad spectrum of applications. The Boost license encourages both
commercial and non-commercial use.

Boost aims to establish "existing practice" and provide reference
implementations so that Boost libraries are suitable for eventual
standardization. Ten Boost libraries are included in the C++ Standards
Committee's Library Technical Report (TR1) and in the new C++11
Standard. C++11 also includes several more Boost libraries in addition
to those from TR1. More Boost libraries are proposed for standardization
in C++17.
