Open Game License
URL = http://ogl.atwiki.com/
From the Wikipedia:
"The Open Game License (or OGL) is an open content license designed for role-playing games. It was published by Wizards of the Coast in 2000 to license their Dungeons & Dragons game as the System Reference Document, or SRD, in a move spear-headed by Ryan Dancey. It is commonly used with the d20 license to allow individuals, amateur and professional companies and groups to publish the SRD and derivative works under the d20 System trademark. It has also been used to license content unrelated to the d20 System and/or the SRD.
Those individuals, groups and publishing companies that license their works under the OGL are sometimes collectively referred to as the "open gaming movement". (http://en.wikipedia.org/wiki/Open_Gaming_License)