This has been addressed before and I know of the possibility of using a library, such as CEGUI, or a widget toolkit such as WX or Qt. Or even combine Java GUI and GL. But what if one wants something else? Are there are other ways to get a nice-quick GUI? Some other libraries?
GLUI is an option. A small Windowing toolkit built over glut. Not as good as Qt, but good enough to serve small applications.
Here is an EXAMPLE
And, i guess its possible to use FLTK with OpenGL as well. i haven’t tried that though.
What if GLUI and FLTK are both out of the question? Maybe cull blenders GUI code?
Anyway, yeah even though GUI toolkits are 2D it is not easy to replace them with some boiler-plate code 
Many non-trivial algorithms play a role there, like automatic layout algorithms. If I find an easy solution I will post again here.