The STLport project is a free, open source implementation of the C++ standard library. Although every modern compiler comes with a more-or-less complete standard library, there remain differences, omissions, and errors in most vendor-supplied libraries. If portability across compilers and platforms is a major concern, you might want to use the same library implementation on all platforms.
You might also want to use STLport for its additional features, such as debug mode, which helps detect programming errors. You can also use the library safely in a multithreaded program; for example, reading from a standard container is thread-safe. Of course, writing to any shared data must be properly synchronized. See the Boost project for help with threads and synchronization.
Also included are extensions to the standard library: hashed containers, singly-linked lists, and ropes (strings that scale well for very large sizes).
See my web page at http://www.tempest-sw.com/cpp/ for links and more information.