Unreal Engine 4, which used to have a $20 monthly subscription fee, is now completely free to develop on. Straight from the website:
Unreal Engine 4 is now available to everyone for free, and all future updates will be free!
You can download the engine and use it for everything from game development, education, architecture, and visualization to VR, film and animation. When you ship a game or application, you pay a 5% royalty on gross revenue after the first $3,000 per product, per quarter. It’s a simple arrangement in which we succeed only when you succeed.
Unreal is a gaming engine. This is the same engine used in popular titles such as the new release of XCOM, Unreal Tournament 3, Thief, Smite, Gears of War, and Bioshiock. The full list can be found on this wikipedia article. The engine has been expanded to include almost every platform available today. This is especially awesome for aspiring Indie Developers, as there's no longer even a monthly subscription barrier to develop using Unreal Engine. Now, Epic Games (Unreal's developer) only wins if you win. You can freely develop to your heart's content, and only pay licensing if you're actually making money. Hopefully, this allows the Indie channel to open up even more while allowing students and anyone curious to try developing for free.