A third-party publisher is simply someone other than the original publisher who publishes a product for an existing game. These publications may be licensed or unlicensed. Some early D&D third-party publishers include The Judges Guild and Mayfair Games. The third party sellers' market became huge with the advent of the OGL and the d20 license, resulting in the glut of d20 products. Dungeons & Dragons Fourth Edition offers the GSL as a route for authorized third-party publishing.