The best damn MMO ever made, via Ruby, Sinatra, and Shoes.
The server lives inside the '/server' directory. To run it, make sure you have the bundler gem installed, then 'bundle install' to get the rest.
Once you have that, just 'rackup config.ru', and your server will be running.
You're gonna need Shoes. Once you've got that, run Shoes, choose "open application", and pick the "client.rb" file. Make sure you have the server running!