GameMaker Studio, the engine used to make Hotline Miami and Spelunky is currently free (normally $50).

Download, install, and let it update.

Edit: A poster mentioned you need to select the “Beta” update option (you’ll see it) to get the option for the free license. That’s the option I picked.

The program will restart and launch with a window with three options: one to use the *free* (feature-gimped) version, one to register for a free studio license, and one to enter a studio license.

It’s the second option. Enter your email. Enter it again, hit register. Key comes to you. Enter the key. VOILA.