BYOND, or Build Your Own Net Dream, is an engine for creating 2D online multiplayer games. It is completely free, and also has a Hub where you can easily share your game.
Pro Community Resources
BYOND's site provides easy access to many resources produced over the years.
Since BYOND has gone through several stages, from 1.0 to 5.0, there are many games and resources still available that are older, so look for the resources and games made around 2013 or later to get the BYOND 5.0 content.
Pro Multiplayer online is easy
BYOND focuses on providing easy multiplayer support as the key feature of their engine. Coding a few lines can already give you a simple game! You will just need to do port forwarding and anybody can join your game, either using the BYOND software, or via their web browser.
Con BYOND has its own coding language
BYOND uses DM (Dream Maker), which is similar to object-oriented C++. The language is very easy to learn, but it is specific to BYOND. This easy to learn language may be perfect for a new or indie developer, but for an experienced coder, using DM may be less powerful than the C++ or Java you may be used to.