RPG Design Patterns
RPG Design Patterns is the wiki version of the book by the same name, a.k.a. Design Patterns of Successful Role-Playing Games. As the name suggests, it's a treatise on the makeup of tabletop roleplaying games, with advice on how to choose various subsystems to achieve a desired effect when creating a new one. A visual language for describing various game mechanics is also defined and used throughout.
As of January 2017, the RPG Design Patterns wiki has been taken offline due to spam and failure to accomplish its mission. The old content can still be found on the Wayback Machine.