This explainer by the EFF on the OGL / D&D fiasco is amazing.
Beware the Gifts of Dragons: How D&D’s Open Gaming License May Have Become a Trap for Creators
The company that owns Dungeons and Dragons (D&D) is planning to revoke the open license that has, since the year 2000, applied to a wide range of unofficial, commercial products that build on the mechanics of the game.
