VSTGL is a simple wrapper-type-thing to let you use opengl for your plugin editors. It inherits from AEffEditor, and provides you with everything you'd normally expect from an opengl window. Note that it does not include any widgets or controls - this is just a basic opengl implementation - it's up to you to decide how you're going to use it. Here's a screenshot of a basic pyramid in the process of rotating (note the antialiasing):
This was taken from within Tracktion on Windows. There is a simple message-based timer class included now to provide repeated screen updates.
The main VSTGL code consists of two files: a VSTGLEditor.h header and the corresponding source file. To use it, you just inherit from VSTGLEditor like you would from AEffEditor (or aeffguieditor if you're used to VSTGUI). The download includes the VSTGL files in an example project, plus doxygen-generated documentation. You can also see VSTGL in action in the Harmonical and Simple Colour Organ plugins.
(see also the MultiGUI page, for an example which combines a VSTGL editor with a standard VSTGUI one)