Probably the most brilliant C++ library that currently exists. Covers lots if stuff that is really missing in language and standard library. Also, a playground by the "big boys". A lot of boost libraries have become C++ standard because of their usefulness.
The official web site of the most widely used C++ library. The Boost C++ Library is contributed to by many people, many of which are part of the C++ standards committee. Boost aims to provide libraries for C++ that are of the same quality or better than the existing or future Standard C++ library with many making it to the official standard.