In Python my favs are lxml and sqlalchemy. Very simple to use and solve the problem of xml and database very easily.

In C++ my fav is the Qt system of libraries. Very nice solutions for structuring an application, cross platform, good memory management, COW if you want it, etc. Every release gets better and better. Oh and boost should get an honorable mention. I really like the ranges library and some of the more esoteric libs it provides.