OGL

OGL, abbrev., p.n. the license offered by Wizards of the Coast that allows gaming companies to produce games and supplements that are compatible with D&D's 3rd Edition and 3.5 Edition using the d20 System.
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Addendum #1: Technically the OGL is not a new concept for D&D. In its early days, TSR did not generally pursue litigation against smaller companies that produced compatible works even when they did not hold an official license. That practice was shut down in the mid 80s as TSR started to aggressively protect its copyrights.
Addendum #2: See the d20 entry for more on the OGL.
Addendum #3: There is a similar license for 4th Edition called the Game System License (GSL). Though they later stated that (as long as it was clear which rules were for which edition), material could be produced for both, companies generally were forced to choose what to print under. Due to the more restrictive nature of the GSL and the lackluster popularity of 4th Edition, some companies have stayed with the OGL, most notably Pathfinder by Paizo Publishing (the former publisher of Dragon Magazine for WotC).