D20 System
The set of "core rules" that forms the heart of Wizards of the Coast's Dungeons & Dragons, Star Wars and D20 Modern product lines. The D20 System rules and basic text are publicly available under the terms of the Open Game License. This has allowed many companies to create their own roleplaying games using the D20 System or a derivative thereof.
--IMAGinES 17:29, 10 Jun 2005 (PDT)