gistfile1.rb

# add to user model
has_many :authentications, :dependent => :destroy

# config/routes.rb
match '/auth/:provider/callback' => 'authentications#create'
match '/auth/failure' => redirect('/users/sign_in')

# config/initializers/omniauth.rb
require 'openid/store/filesystem'

Rails.application.config.middleware.use OmniAuth::Builder do
  if Rails.env == "development" || Rails.env == "test"
    provider :facebook, 'YOURSHITHERE'
    provider :twitter, 'YOURSHITHERE'
  end 
  
  if Rails.env == "staging"
    provider :facebook, 'YOURSHITHERE'
    provider :twitter, 'YOURSHITHERE'
  end

  if Rails.env == "production"
    provider :facebook, 'YOURSHITHERE'
    provider :twitter, 'YOURSHITHERE'
  end
end

# app/models/authentication.rb
class Authentication < ActiveRecord::Base
  belongs_to :user
  
  scope :facebook, where(:provider=>"facebook")
  scope :twitter, where(:provider=>"twitter")
  
  def provider_name
    if provider == 'open_id'
      "OpenID"
    else
      provider.titleize
    end
  end
end

#
# Table name: authentications
#
#  id         :integer(4)      not null, primary key
#  user_id    :integer(4)
#  provider   :string(255)
#  uid        :string(255)
#  created_at :datetime
#  updated_at :datetime

# app/controllers/authentications_controller.rb
class AuthenticationsController < ApplicationController
  def create
    omniauth = request.env["omniauth.auth"]
    
    if !omniauth
      redirect_to new_user_registration_url
    elsif authentication = Authentication.find_by_provider_and_uid(omniauth['provider'], omniauth['uid'])
      # we already have an authentication
      # sign the user in that it belongs to
      flash[:notice] = "Signed in successfully."
      sign_in_and_redirect(:user, authentication.user)
    elsif current_user
      # we are adding a 3rd party solution to an existing user
      current_user.authentications.create!(:provider => omniauth['provider'], :uid => omniauth['uid'])
      flash[:notice] = "Authentication successful."
      redirect_to profile_url
    else
      @user = User.new
      @user.apply_omniauth(omniauth)

      if @user.save
        flash[:notice] = "Signed in successfully."
        sign_in_and_redirect(:user, @user)
      else
        session[:omniauth] = omniauth.except('extra')
        redirect_to new_user_registration_url
      end
    end
  end

  def destroy
    @authentication = current_user.authentications.find( params[:id] )

    if @authentication.destroy
      redirect_to profile_path, :notice => "You have disabled #{ @authentication.provider_name } authentication."
    else
      redirect_to profile_path, :notice => "You can not delete this authentication."
    end
  end
end