Ryan Dancey was the main advocate of the Open Game License and D20 SRD at Wizards in 1999-2000. On this forum post at the Paizo Messageboard he reflects on it's legacy. The release of the d20 SRD under the OGL is a seminal event in the history of Dungeon & Dragon and I thank Mr. Dancey for it. It is also the foundation of countless games including the various retro-clones we now happily play.
One of the things that makes the OGL well suited for what we do is that works can have Product Identity. Product Identity are elements of a work that are unique to the publisher and allows each of use to protect what is uniquely ours but still keeps the common rules elements free for anybody to use.
Dungeons and Dragons and Philosophy: The Book
25 minutes ago