A quick scan of Wikipedia suggests :
OpenGL - 1992
Brian Paul starts working on Mesa - 1993
Direct3D - 1995
First HW acceleration in Mesa - 1997
Common use of term "open source" - 1998
Jens Owen & Kevin Martin write initial DRI design doc - 1998
John Carmack donates $10,000 to Mesa project - 1999
Mesa is far from being compliant with the newest OpenGL specification, and as such is in no way a "gold standard for correctness" either. That makes OpenGL an "open" specification with only closed implementations. I could even go so far as to say that I wouldn't consider OpenGL really open until there is an actual open reference implementation out there.
In other words; I'm going to be playing my games with the proprietary Nvidia driver if I want to enjoy the latest version of this specification.
By the way, I said a free implementation, didn't I? I believe the term free software dates back to the early 80's, if not earlier.
There was an "official" reference implementation from SGI (which SGI open-sourced recently) but the last update was to OpenGL 1.2.
BTW before you pick a specific proprietary implementation as your "reference", you might want to read up on strict vs relaxed implementations. There are some significant differences in the way that out-of-spec API calls are handled, each with pros and cons.
I was going to say something like "they didn't call it FreeGL" earlier, but I thought that could have come across as more sarcastic than I intended ;)
"""The Khronos Group today announced the immediate release of the OpenGLŪ 4.3 specification, bringing the very latest graphics functionality to the most advanced and widely adopted cross-platform 2D and 3D graphics API. OpenGL 4.3 integrates developer feedback and continues the rapid evolution of this royalty-free specification while maintaining full backwards compatibility, enabling applications to incrementally use new features while portably accessing state-of-the-art GPU functionality across diverse operating systems and platforms. The OpenGL 4.3 specification contains new features that extend functionality available to developers and enables increased application performance. The full specification is available for immediate download."""
OpenGL is a Royalty-Free specification. They will not give you their closed code, but you can write your own that works similar, without the concern for patents. Free specification (how it works), not free code. Of course there are side patents like S3TC, but OpenGL will destroy them eventually with new open standards.
I don't think there will be another important D3D version.