In a posting on the EN World message boards, Wizards of the Coast Brand Manager Scott Rouse has stated unequivocally that as far as 4th Edition D&D is concerned: 'There will be the OGL (Open Game License) and Wizards D&D products only. [There will be] no d20 STL (System Trademark Licensing) tiered or otherwise.'
When Wizards of the Coast introduced the OGL in 2000, WotC also inaugurated a program that provided a separate license--the d20 System Trademark License (STL)--that allowed third party publishers to use some of WotC's trademarked terms and a distinctive logo to identify these products (see 'd20 and the Open Gaming System Explained'). All d20 STL products were required to be tied to a D&D core rulebook (published by WotC) and were also required to acknowledge the requisite core rulebook clearly.
The introduction of the OGL and the d20 STL led to the establishment of new companies specifically focused on producing d20 role-playing content, resulting in a glut of products hitting the RPG market at a time when the market was under increasing pressure from various forms of electronic RPGs and more recently MMORPGs (massively multi-player online role-playing games). In the last few years the number of products from third party publishers using the d20 STL has been declining, so the effect of WotC's pulling the plug on the d20 STL is uncertain.