Useful C++ libraries.
Boost
Probably the most important set of libraries other then the STL, Boost offers a huge amount of functionality. Libraries featured in Boost may eventually be included in future C++ standards.
Xerces C++
Xerces is a validating XML parser with support for Dom, SAX, XML Namespaces and XML Schemas. This library is provided by the Apache foundation.
Xalan C++
Another library from the Apache foundation, Xalan is an XSLT processor, compatible with Xerces.
wxWidgets
wxWidgets is a lightweight cross platform GUI framework. Unlike some widget kits it uses the native widgets of the current platform. This gives it a more native feel and makes it fast.