Yokosuka 2k is a sprite-based beat 'em up game engine in the style of games like Final Fight, Streets of Rage, and River City Ransom.
At the moment, it is written in JavaScript and runs in your browser, although there are plans to get it working on other platforms as well.
I've tried to design Yokosuka 2k in a functional style to keep the design clean and simple.. I've enjoyed designing it and plan plan on writing several blog posts explaining the design in more detail.
You can check out a preview for now. In the future, I'll publish more examples of games written using it.
I plan on open-sourcing the various parts of the game engine so you can use it to build your own beat 'em up game. At the moment, you can take a look at Animata, the finite state machine used to drive the actors and game animations, on github.
To contact me, email ussherpress (at) gmail (dot) com or tweet @ussherpress.