As far as I know it is like this:
In *nix world you get: Mesa (up to 3.1 spec), proprietary GPU driver's implementation (up to 4.3 for nVidia, 4.2 for AMD) and OSS driver's implementation (i have no idea, enough to play Quake3 based games).
In Windows you get: Microsoft implementation (1.1 spec) and GPU driver's implementation (up to 4.3 for nVidia, 4.2 for AMD).
In OSX you get: Apple's implementation (up to 3.2 for everyone).
EDIT: Corrected Mesa's and Apple's spec implementations.