At the Weapons Options screen, move the cursor away from the text area and press C, Z, Z, C, Z, Z, C, Z, Z. If that doesn't work, press C + Z nine times.
For a christmas map, enter the password JINGLEBELLS.
For the ultimate tournament map, enter the password "00035907". This map is entirely flat, making it a great for tournament play.
To get a random Hell level, enter "666" as a seed for the landscape generator.