The Standard Edition of GameMaker: Studio is free for a limited time, according to a post on the official GameMaker Studio blog. Normally the game engine software retails for $50, but the company is giving it away for free. It's the same set of tools used to make such games as Hotline Miami, Gunpoint and the original Spelunky.
To get it you'll have to jump through a few hoops (detailed on the blog), but if you are an aspiring developer or have been toying with the idea of making a game, it's worth those steps. You may also have a hard time downloading the software because the volume of traffic hitting the download site is pretty high at the moment.
Find out all you need to know to get it here.