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 already included in the
C++ Standards Committee's Library Technical Report (TR1) as a step
toward becoming part of a future C++ Standard. More Boost libraries
are proposed for the upcoming TR2.
