Useful C++ libraries.
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 is a validating XML parser with support for Dom, SAX, XML Namespaces and XML Schemas. This library is provided by the Apache foundation.
Another library from the Apache foundation, Xalan is an XSLT processor, compatible with Xerces.
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.