GMLive is an extension that introduces livecoding / interactive programming into GameMaker: Studio and GameMaker Studio 2 (including Mac IDE).
That is, it allows to reload chosen scripts or events mid-game when they are changed and saved from the editor - without having to recompile and restart the game.
This allows to save tremendous amounts of time during development, especially for projects and platforms with longer compile time.
The extension works with all native modules - you can even have the code update remotely while the game's running on a mobile device or a console.
The extension is stable, documented, easy to use, and takes literal minutes to set up (see video).
Note: since the initial release, the extension was also expanded to support sprite and room reloading!