Devise is a flexible authentication gem.
Installation
Rails 3: Add the following to your Gemfile
gem "devise"
gem "hpricot"
gem "ruby_parser"
Install devise in your project
$ rails generate devise:install
Generate devise for your model
$ rails generate devise MODEL
$ rake db:migrate
(Optional) Generate devise views
$ rails generate devise:views
Helpers
user_signed_in?
current_user
user_session
destroy_user_session_path (Logout)
new_user_session_path (Login)
edit_user_registration_path (Edit registration)
new_user_registration_path (Register new user)
Controller stuff
before_filter :authenticate_user!
Model
Model options
class User < ActiveRecord::Base
  devise :database_authenticatable,
    :registerable,
    :confirmable,
    :recoverable,
    :rememberable,
    :trackable,
    :validatable
end
Migration helpers
create_table :users do |t|
  t.database_authenticatable
  t.confirmable
  t.recoverable
  t.rememberable
  t.trackable
  t.timestamps
end
Routing
Authenticated and unauthenticated routes
unauthenticated do
   root :to => 'home#index'
end
authenticated do
  root :to => 'dashboard#index'
end
As
as :user do
  get 'sign_in', :to => 'devise/sessions#new'
end
Devise_for magic
devise_for :users
    # Session routes for Authenticatable (default)
         new_user_session GET  /users/sign_in                    {:controller=>"devise/sessions", :action=>"new"}
             user_session POST /users/sign_in                    {:controller=>"devise/sessions", :action=>"create"}
     destroy_user_session GET  /users/sign_out                   {:controller=>"devise/sessions", :action=>"destroy"}
   
    # Password routes for Recoverable, if User model has :recoverable configured
        new_user_password GET  /users/password/new(.:format)     {:controller=>"devise/passwords", :action=>"new"}
       edit_user_password GET  /users/password/edit(.:format)    {:controller=>"devise/passwords", :action=>"edit"}
            user_password PUT  /users/password(.:format)         {:controller=>"devise/passwords", :action=>"update"}
                          POST /users/password(.:format)         {:controller=>"devise/passwords", :action=>"create"}
   
    # Confirmation routes for Confirmable, if User model has :confirmable configured
    new_user_confirmation GET  /users/confirmation/new(.:format) {:controller=>"devise/confirmations", :action=>"new"}
        user_confirmation GET  /users/confirmation(.:format)     {:controller=>"devise/confirmations", :action=>"show"}
                          POST /users/confirmation(.:format)     {:controller=>"devise/confirmations", :action=>"create"}
Customizing devise_for
devise_for :users,
  :path => "usuarios",
  :path_names => {
    :sign_in => 'login',
    :sign_out => 'logout',
    :password => 'secret',
    :confirmation => 'verification',
    :unlock => 'unblock',
    :registration => 'register',
    :sign_up => 'cmon_let_me_in' }
Test helpers
include Devise::TestHelpers
https://github.com/plataformatec/devise/blob/1094ba65aac1d37713f2cba71f9edad76b5ca274/lib/devise/test_helpers.rb
sign_in @user
sign_out @user