Documents which seem to be hard to find
While the vast majority of OpenGL extension specifications are available online in the extensions registry, some can be quite hard to find.
Sections
WGL_3DFX_gamma_control extension
S3_s3tc extension This is the specification for the S3_s3tc extension, which allows programmers to use S3TC compression in OpenGL. The original document seems to have been removed from the S3 site, although there is a description of the extension at the bottom of this page, so this is a copy of the specification with the links to other parts of the (old) S3 site removed, and the rest unaltered.
View the extension specification
WGL_3DFX_gamma_control extension This is a description of the WGL_3DFX_gamma_control extension, used to set gamma ramps on 3dfx boards under Windows. Note: I was unable to obtain a specification for the extension anywhere, so this document is simply my description of how I believe it works. It may be worth mentioning that I have actually got it to work in game code.
View the extension specification
WIN_swap_hint extension WIN_swap_hint, which can be used to improve the performance of buffer swap operations on Windows by specifying that only part of the window actually needs to be swapped, is documented by Microsoft, but the documentation actually seems to exist as an MSDN entry under the name of the function exported by the extension (glAddSwapHintRectWIN) rather than a "normal" extension specification. Possibly as a result, this extension does not appear to be documented in the OpenGL extensions registry, and many people have trouble finding the documentation. So here is a mirror of the online documentation for glAddSwapHintRectWIN, from the Microsoft site.
Note that WIN_swap_hint generally only significantly boosts buffer swapping performance on drivers where the actual swap is implemented as a software copy.
View the extension specification
SGIS_multitexture extension SGIS_multitexture was the first OpenGL extension which supported multitexturing. PLEASE NOTE: THIS EXTENSION IS NOW OBSOLETE, AND DEPRECATED IN FAVOUR OF ARB_MULTITEXTURE (see appendix F to the OpenGL 1.2.1 specification, downloadable from www.opengl.org). I strongly suggest that you do not use this extension in new code, since it will not continue to be supported in future drivers, and the ARB_multitexture syntax is in any case superior to that of SGIS_multitexture.
However, there are some good reasons for wanting to look at the specification, e.g. if you are trying to convert old open source code from using SGIS_multitexture to ARB_multitexture. I've therefore mirrored this specification here from the SGI site since, as a deprecated extension, its not listed in the OpenGL extensions registry, and people seem to find it hard to track down.
Note also that EXT_multitexture has the same specification as SGIS_multitexture (the only difference is that it was named as a general extension rather than an SGI specific one). EXT_multitexture is therefore also obsolete and, like SGIS_multitexture, should not be used in new code.
View the extension specification
KTX_buffer_region extension KTX_buffer_region is an extension maintained for use by the Kinetix 3D Studio Max family of Windows programs only. It is not documented in the the OpenGL extensions registry, and its functionality is not necessarily guaranteed to remain the same in future driver releases. Its use in games is therefore not encouraged. But, in case you have a desperate need for an offscreen buffer of the type offered by this extension, and cannot use Windows pbuffers, here is a mirror of the documentation from Don Brittain's excellent webpage on the 3D Studio Max pipeline.
Note that even if the extension is exported by the driver it is not guaranteed to be usable: see the notes on the glBufferRegionEnabled function in the document linked below.
View the extension specification