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

The emphasis is on 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.

The goal is 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) and will be in
the new C++0x Standard now being finalized. C++0x will also include
several more Boost libraries in addition to those from TR1. More Boost
libraries are proposed for TR2.