OpenGL

• XFree86 3D Status Report (the free X Server for linux)
• OpenGL Implementations on X, Mac, Linux, OS/2, etc.
• Mesa (old link)
• GLX module for Mesa-3.0 and XFree86

The GUI Toolkit, Framework Page

• GLUT 3.6
• GTK
  • Documentation
  • Download
    • Stable series 1.0.x
  • GTK+ Application Repository
    • GtkGLArea (GTK+ Widget for OpenGL drawable area)
• FLTK (alternate link)
  (a very easy GUI based on the old Forms package---I just heard about this so I haven't installed or played with it yet, but this may be easier to use than gtk, should be much more robust than glut and should even have a visual GUI creator which will let you set up buttons, windows, etc., and generate the skeleton callback C file, or at least that's how the old Forms worked).
  • On-line documentation ( alternate link)