OGL

OGL or Open Gaming License is a open content license for roleplaying games. It was released by Wizards of the Coast with the System Reference Document (the basic game mechanics) for D&D 3rd edition.

This allowed other companies and users to develop settings, adventures and so on in the d20 system that came with 3rd edition.

D&D 4th edition has seen a reversal back to a more closed and limited license which practically prevents people for releasing third party material for it without paying Wizards for licensing.