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BYOND, Build Your Own Net Dream, is an engine that specializes in 2D multiplayer online games, although it is also excellent for single player games. It is 100% free, no strings attached. You can even easily share your game using the BYOND Hub, or release your game separately on any other distribution site.
BYOND Beginner Guide: http://www.byond.com/docs/guide/
BYOND Code Reference: http://www.byond.com/docs/ref/
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Pros
Pro Abundant features
BYOND handles all networking for you, and provides you an environment for editing code, and even a built-in map editor and image editor. An interface editor for customizing your window's interface is even included. Mouse and keyboard input are easy to customize. BYOND also has default procedures that handle collisions, basics of pixel movement, and many other procedures at your disposal.