Tardvig 
No longer developed. I do not recommend to use it. (see the tarona readme)
Description
Lightweight structure for Ruby-based game engines.
In fact, this is just a bundle of several simple helpful classes and modules which can be integrated and extended to make a game engine.
What is it for?
It was developed for my single-player browser game which is divided into levels, but probably this gem can be used for any other types of games with any IO, I tried to make it as flexible as possible. For example, I used a similar structure for my text-based game with console IO earlier.
Important! I recommend to familiarize with this gem before using it because it may be not applicable to your game.
Getting Started
Tardvig consists of some modules and classes (i will call them "parts"). In this guide I am not going to describe them, because you already have docs as their description and specs as their examples. I'll tell you what you should pay attention to. I recommend you to read the description of parts immediately after you noticed their names so you can understand further guide (however, the guide is short).
Events mixin and Command class are base parts which are not related directly
to the game structure, but are very useful for parts which are related to it.
The main part of a Tardvig-based game is Act class. Game is a sequence of
acts. Acts can have different types.
Example: Tardvig-based 3D FPS would be a sequence of acts of types shooting
(user can run and shoot; each location is an act) and cutscene (user watches
video), finished with a credits act (user watches authors' names).
Important fact is that acts have attribute subject.
Acts have attribute io which should contain an instance of any class which
gives you ability to communicate with something that will dislay your game.
I recommend you to communicate via GameIO.
Well, now you know the basic parts. Final thing you should to do before you can
use this gem is reading the example. It takes place in the
spec/integration_spec.rb file.
You can also see...
Rakefile - Rake tasks for contribution
tardvig.gemspec - here you can see my name and email and this gem's depencies
LICENSE - this gem's license