The Dungeon Contest probably is better known by the colloquial name "one page dungeon" contest. The contest has been run annually since 2009. Annual requirements have varied slightly over time, but in general entries must conform to these restrictions:
Submitted as an unlocked PDF
Consist of a single page of A4 or US Letter (one sided)
Include a "Creative Commons Attribution-Share Alike 3.0" license
Have a title and at least one credit
Be original work
Be system neutral
Because the submissions are available for use under a liberal license, they often are gathered into collections and published in various formats by different people. The contest organizers have made collections available for free download, but various third party collections in print and electronic format have also been offered for purchase.