Containers to extend STL: graph, tree, matrix, smart-pointer. Persistence (serialisation) of C++ data structures. Portability: subprocesses, TCP, file system. Reusable subsystems: library manager, message handler. Portable between OSs and compilers.
Documentation is included in the download and is also available on the STLplus website http://stlplus.sourceforge.net. The headers are intended to be readable, so look at them too for usage information on each function and class.