Quick links: Documentation · itch.io page (cheaper; the demo's there)
Versions: GameMaker: Studio, GameMaker Studio 2
Platforms: Windows, Windows (YYC)
This extension allows you to take input from multiple mice and keyboards! This way you can differentiate between mouse movement and button presses on mice (and/or touchpads) on the same computer, or map controls to same keys on different keyboards, or work with other mouse/keyboard-like devices.
I hope this to be an invaluable tool for anyone making local multiplayer games.
Implementation-wise, this uses Raw Input API (MSDN) along with a bit of helper code by Jake Stookey (link; my changes) for mouse polling, and the usual tricks to get around the GameMaker's extension system limitations.
End User Licence Agreement (EULA).