Qt is a really nice cross-platform gui lib, some consider it to be one of the best examples of how to write an API.
Its written in C++, but has bindings for other languages. It is very mature and robust, has lots of nice tools, and recently was purchased by Nokia, who actively support development.
It is also the platform on which KDE is built.
I've played around with it, the developers seem like very capable people, and the capabilities of the library seem top-notch, but I haven't done any non-web development for a while so I haven't had a chance to really exercise it and see how I like the workflow.
