User on OpenGL.org forum posted results of his testing of about 2000 user PCs and their GPUs. The result is nice online database of hardware features. This allows low level OpenGL coders to check (using José's glext.inc), whether the features they use will run on hardware of their customers before shipping the product for example.
Website also allows performing feature support comparisons between multiple GPUs. A must have reading:
http://feedback.wildfiregames.com/report/opengl/
Petr