the more difficult part is the event handling of parent/child controls. how they propagate them and how its handled in general. once you designed your own solution for that anything else will be just rendering quads and/or text. and yes, satisfaction guaranteed!
Pretty sure FLTK does not render widgets with OpenGL (it is LGPL though which is convenient). Honestly though if you want a very nice GUI toolkit that “doesn’t” use OpenGL for rendering and is LGPL or better (i.e. freely usably for personal/commercial), use Qt 4.5 (the LGPL is a very pleasant change; thanks Nokia!).
But for GUI toolkits that do use OpenGL for widget rendering, google “opengl gui libraries” or “opengl gui toolkits”. You’ll get a bunch of hits. No personal experience with any of them.
Well, if you are happy with CEGUI that is fine. My problem is always that I want something extremely light weight and free from other dependencies.
Which you probably have come across is really simple and fits the bill - even if it is not very pretty and perhaps not actively maintained. However, I have found that in many cases, it is preferable to have a console. It is a well kept secret, but