GMare is an open source alternative room editor for the RAD tool Game Maker (Versions 5 to Studio). Offering a robust toolset to create rooms quickly and easily. It also offers various import and export options.

Goes without saying, the development of GMare was stopped soon after GMS 2 came out, and they made a decent room editor for a change. Not all the bells and whistles I would've liked to see (At the time at least), but it was a definite improvement. I was working on another version of GMare at the time, that allowed automatic tiling. Maybe I will upload that source at some point.

GMare 1.0 Screens:
Area Map Option
Tile Color Blending
Shortcuts

Ahhh, the dark theme and the cumbersome ribbon control of GMare 2.0 (For giggles):
GMare 2.0 Screen 1
GMare 2.0 Screen 2