Libraries & tools: Various libraries and utilities. Look here if you want to compile the other demos on this site yourself. At the very least, you will need to download the DOT.
OpenGL essentials: Demos of the fundamental concepts of OpenGL programming.
Texturing: Demos that show various texture mapping techniques and effects.
Lighting and shadowing: Demos of various lighting and shadowing techniques.
Speedups: Demos of various techniques to speed up your rendering.
Buffer stuff: Demos of techniques that operate on the framebuffer in interesting ways.
Geometry: Demos that perform all kinds of geometry or polygon manipulation.
Special effects: Eye candy and advanced rendering techniques.
Miscellaneous: Demos that don't fit into any other category, e.g. DirectX-related or 3D sound.