BYOND is a free 2D game engine for making and playing online multiplayer games. With BYOND's all-in-one IDE, an easy-to-learn object-oriented programming language, and baked in large scale multiplayer support, it's never been easier to share your dreams with the world.
BYOND is 100% free, and is excellent
Richard Bartle, father of the MUD
Writing games in BYOND is fast and easy. Write object-oriented code in BYOND's own DM, a flexible, but powerful programming language.
BYOND's renderer is ready to do all the work needed to make crisp 2D isometric, top-down, or side-scrolling games.
The Dream Maker IDE contains a code editor, an animated sprite editor, and a map editor so you don't have to juggle a bunch of applications to make something great.
BYOND is multiplayer capable out of the box. There is no need to write messy networking code. It just works!
Release your game on BYOND's platform, or to the wider world. BYONDexe features Steam integration, and the HTML5 web client allows you to share your project on the web.
If you get stuck, don't worry, plenty of people are here to help! You can also browse a whole pile of resources and tutorials.