Boost provides free peer-reviewed portable C++ source libraries. Many of which will be part of the new C++0x standard when it is released some year.
"This book is an introduction to the Boost C++ Libraries which complement the C++ standard by adding functions helpful in practice. As the Boost C++ Libraries are based on the C++ standard, they are implemented using state-of-the-art C++. They are platform independent and are supported on many operating systems including Windows and Linux by a large developer community.
The Boost C++ Libraries enable you to boost your productivity as a C++ developer. For example, you can benefit from smart pointers which help you to write more reliable code or use one of the many libraries to develop platform independent network applications. Since many Boost C++ Libraries are going to be incorporated into the next version of the C++ standard, you can prepare yourself starting today."
I use the Boost libraries extensively myself when developing Windows or Linux based code. Alas I've yet to find a C++ compiler that I'm happy with that will run on small embedded micros, that would allow me to take advantaged of Boost.