Orx and Qt note.
The problem with QT (and a lot of other widget libraries) is it wants control of your soul–er, the entire main program loop. It wants to set up the OpenGL context itself and have full control of everything.
I also looked into this when I was evaluating widget libraries and thought it looked like too much trouble for me (then again, I’m not so experienced with OpenGL).
There are a few widget libraries designed to render into an existing OpenGL context and get out of your way. Of these, I’ve successfully got both libRocket and CEGUI working very well in Orx. Check out my other thread on libRocket if you’re interested in using it, since some changes need to be made to the rendering code.