multiplayer-oriented maps for Doom
Freedoom is a project to create a complete Doom II-compatible IWAD file which is Free Software.
FreeDM is a sub-project to create a set of levels designed for deathmatch play.
FreeDM does not require Freedoom to play and can be used in conjunction with a boom-engine. Please note that this is designed for multiplayer and will not be of much use for single player.