Here is where participants can discuss modding resources for games.  Modding an existing game can often be a better and more efficient option in a game jam than creating a new one from scratch or with a general purpose engine, and plenty of popular game franchises, from Counter-Strike to Dota to PUBG, began as mods.  Mods are every bit as acceptable in this jam as original games.

DOOM (1993)

Eevee’s 3 part tutorial series “You should make a Doom level!” is a good introduction for beginner mappers.  See also the Editing Tutorials subforum on Doomworld and JP LeBreton’s Doom level editing tutorial.

Editors: SLADE (Windows/OSX/Linux), DoomBuilder X (Windows)

American Truck Simulator / Euro Truck Simulator 2

Resources: SCS Software Official Modding Wiki

Quake 1

Resources: Dumptruck_ds has a helpful Youtube series on Quake mapping with the Trenchbroom editor.  Many of the same concepts are applicable to later id Tech 2 and 3 games like Quake 2 and 3, Hexen II, and Daikatana.

Editors: Trenchbroom, J.A.C.K (also supports Half-Life below)

Half-Life

Resources: Valve Official Documentation, see also the tutorials section on Mod DB and the /r/hammer subreddit.

Editors: Hammer, Sledge

Minecraft

Resources: Mr. Crayfish Youtube tutorials, Forge documentation

Cities: Skylines

Resources: Skylines wiki and r/CitiesSkylinesModding

Hotline Miami

Resources: See Hotline Miami fan wiki

Civilization VI

Resources:  See Civ 6 Fandom wiki