I remember once I needed to macro up a software that needed an X server with OpenGL to run. (Like really dirtily hack it up, with xmacro and stuff like that). I wish I could set up a headless X server with a dummy OpenGL renderer (witch doesn't actually render anything), so it doesn't bottleneck on rendering that isn't used anyway. I guess it's even easier to write such and OpenGL implementation.

edit: Now I see it doesn't implement the OpenGL API though, the goals are obviously different.