Take a look at FLTK. It can be used for all sorts of things and repurposed very simply. It is used especially for application building but is perfectly capable of handling 3D objects and built is on OpenGL.

Personally I just have a buffer for menu backgrounds, button textures, navigation and string/placement info + an input handling library. Cheers.

The creators may shoot me for saying so, but I found GLFW to be just awesome at creating cross-platform GUI frameworks. The GLFW development folks go on and on about GLFW does not handle GUI. It does however do a nice, simple, clean and totally portable job of handling graphics, events and mouse/keyboard/joystick tasks.

The nice thing about GLFW for me is that it is fairly small, requires no system specific code, no GLUT and is well maintained. Has been stable for use on OpenGL 4.1 projects so far. Compared to something like SDL I love the tiny overhead and it was easy as pie to write GUI handler classes.

Alternatively, take a look at FLTK. It can be used for all sorts of things and re-purposed very simply. It is used really for application building but is perfectly capable of handling 3D objects and is built on OpenGL. Very small, well supported and while I don't see many people making custom interfaces on the game design level, there are some that seem to work great.