As a C++ programmer I think that the Boost library would a a good candidate for
Its particularly good at filling some of the holes in the C++ standard library,
and is in written in part by some of the C++ standards group.
Its also cross platform, so I can use it without fear that I won't be able to
run code on Windows.
Let me double in on that. Boost gives me portable threads, a nice python
interface, a top-down parser, etc., etc.