TheCoreWithin

Qt 4.5 came out a few weeks ago, the first LGPL Qt!
http://www.qtsoftware.com/products

I've played with an earlier version and used it in my masters thesis. I can't say that it's as elegant as Cocoa, but it came close, and it is as platform independent as a GUI toolkit can get so far (unless you count Java).

I liked it quite a bit though.

Boost is a C++ sourcecode library. It offers many basic tools like (my favorite) std::tr1::shared_ptr, functors and many more.
http://www.boost.org/

If you want to take a look at the future of the language, this is the place to go. (TR1 will be added to the C++ standard sometimes)