Greyhawk

Greyhawk is the earliest and probably the most popular (with hardcore gamers) Dungeons & Dragons world to play in. Greyhawk is the name of 'the Jewel of the Flannaess', a free city originally under the domination of the Kingdom of Aerdy[?], home to the mad wizard Zagyg, one of Oerth's aspirant demigods, which helped it attain its present status.

Flannaess is the continent much of Greyhawk action takes place on, and the planet is called Oerth.