routes.rb

ActionController::Routing::Routes.draw do |map|
 map.twitter 'twitter', :controller => "twitter", :action => "twitter"
 map.root :controller => "twitter" 
end

twitter_controller.rb

class TwitterController < ApplicationController
  before_filter :setup_consumer, :only => [:index, :twitter]
  
  def index
    # First step: get a request token.
    # Remember to specify the oauth_callback option so twitter knows to redirect back to your app.
    # If you don't, then it will give you a PIN and that is *so* 1990s.
    @request_token = @consumer.get_request_token :oauth_callback => twitter_url
    
    # Store the oauth_secret in the session.
    # We use it when Twitter redirects back to us.
    session[:oauth_secret] = @request_token.secret
    
    # Redirect out to Twitter to let the user choose to authorize us.
    redirect_to @request_token.authorize_url
  end
  
  def twitter

    # Twitter redirects back to this action.
    
    # We setup another request token.
    @request_token = OAuth::RequestToken.new(@consumer, 
                                               params[:oauth_token], 
                                               session[:oauth_secret])

    # We verify that this request token is 100% legit (or there abouts) by using oauth_verifier
    @access_token = @request_token.get_access_token :oauth_verifier => params[:oauth_verifier]
    
    # Now that we're authenticated, let's get the user's information just for the sake of it.
    @credentials = @access_token.get('/account/verify_credentials.json')
    
    # Or we could post a message by doing this:
    @access_token.post('http://twitter.com/statuses/update.xml', { :status => "If this works, then I am authenticated with Twitter through OAuth"})
    
    # Done!
    render :text => "Twitter has been spammed!"
  end
  
  private
  
  def setup_consumer
    # This is not really my key and secret (I'm not *that* stupid), just random characters.
    # Setup the consumer here.
    @consumer = OAuth::Consumer.new("Nn1QencWdwK7Dh8sEdoA", #Consumer key
                                   "4M76Ct1nrSZPkO42YHF0dSdmk57lKChQ0GmeJirs", #Consumer secret
                                   :site => "http://twitter.com") # Yes, OAuth isn't just for Twitter!
  end
end