I've done one or two useful C++ things in my time. The publicly available stuff is here. (You might also find the software page interesting).
An automated build system that will build the excellent Boost libraries as an Apple framework. Find out more about it here.
Find it on gitorious here.
(Somewhat old) A C++ debugging support library.
It's available on sourceforge here.
(Old) A library providing a STL-style circular buffer container. It accompanies a set of articles I wrote for Overload magazine.
Find out more here.
Check out my blog, where I developer a more recent version of this. Please use that version in preference to this old one!