I know that title might seem a bit over the top but hear me out. YellowAfterLife (YAL) has put together what I would consider as one of the most useful and easily the most groundbreaking extension to date for Game Maker. GMLive!
Realtime code changes
What this means is that you can edit code without having to recompile the game. I threw together an example video for YAL which you can watch here. It is really quite incredible. If I had this tool available back when I was working on Rock God Tycoon I would've been able to create the game in half the time.
This extension has really given me a fresh breath of air and I'm excited to work in GM once again!