Faye is a pub/sub messaging server (plus client) for web. If you need realtime information processing & dessimination capabilities in your web app (something like a chat application), Faye is what you can use.
New to Faye & do not know how to use it in Ruby ? This is a good place to start.
Install dependencies
gem install faye eventmachine thin
Fire up Faye server
rackup faye.ru -s thin -E production
Fire eventmachine client which will respond to the browser request
ruby eventmachine.rb
Open client.html in browser (since you do not have a webserver running, you can simply open up the file in browser to execute javascript)
The client will send a message & eventmachine.rb will respond to it. Both of the parties are suppose to respond instantly to the incoming message.
Now that you have seen Faye in action, go build your own realtime messaging app in Ruby/Rails using Faye.