GLAF is a lightweight library written in portable C code, designed to help OpenGL developers who need to access extensions or advanced features from applications that are supposed to run unmodified across a wide range of OpenGL versions.

Developers who want that their applications be compatible with older OpenGL versions, usually need to do some unpleasant work: The same feature may be implemented either as an extension or as a core feature (or both) depending on the OpenGL vendor and version. To make things worse, accessing an extension implies extra work: Determine availability with string searching methods, and querying pointers to the extension functions (to query pointers is often ignored in UNIX, but you really need to do it if you don't want that 'rld' fails when your application starts up in a machine whose 'libGL.so' lacks functions that your application calls).

GLAF does all this work automatically for you. It looks for the best way of accessing a given feature in your OpenGL implementation (the preference order is to try using core features first; if that fails, ARB/EXT extensions; and then, vendor extensions).
 

GLAF is open source

It's distributed as open source, and it's free (MIT license) for both commercial and non-commercial developers.