I'm quite surprised to see that nobody has mentioned fltk (http://fltk.org). It is light, easy to use, and it uses a modified gpl license that allows linking against non-free code. And it has native look and feel too...
IIRC, FLTK had their 2.0 version in the works a long time ago ... and it just kept being in the works. They just never left their 1.x series. Was their redesign doomed to never replace 1.x?