glGUI
... cross plattform, API independent GUI
NeHe ... Jeff
Molofee's great OpenGL tutorial page
Lonerunner
... more great examples
Hugi27
... Drawing ActiveX Controls into Memory Buffer (source)
Christian
Geiger ... my professor
Medieninformatik
... my course of studies
Dig-ID
... OpenGL presentation engine, a former project where I took part
ARWin
... Desktop AR Window Manager. These guys got the cool name ;)