all_in_controller.rb

# Typical way of handling the email in the controller
class UsersController < ApplicationController
  def create
    @user = User.new params[:user]
    UserMailer.deliver_welcome(@user) if @user.save
  end
end

ar_observer.rb

class UserObserver < ActiveRecord::Observer
  def after_create(user)
    UserMailer.deliver_welcome user
  end
end

callbacks.rb

class UsersController < ApplicationController
  resource_controller
end

class User < ActiveRecord::Base
  after_create :deliver_welcome_email

  def deliver_welcome_email
    UserMailer.deliver_welcome self
  end
end

intention_revealing_method.rb

class UsersController < ApplicationController
  def create
    @user = User.new params[:user]
    @user.signup
  end
end

class User < ActiveRecord::Base
  def signup
    UserMailer.deliver_signup(self) if save
  end
end

observer_pattern.rb

# Using the observer pattern (rather than Rails observers)
class UsersController < ApplicationController
  def create
    @user = User.new params[:user]
    @user.add_observer(:created => lambda {|u| UserMailer.deliver_welcome(u) })
    @user.save
  end
end

service.rb

# Using a service to remove duplication
class UsersController < ApplicationController
  def create
    @user = User.new params[:user]
    UserService.signup @user
  end
end

class Admin::UsersController < AdminController
  def create
    @user = User.new params[:user]
    UserService.signup(@user)
  end
end

class UserService
  def self.signup(user)
    UserMailer.deliver_welcome(user) if user.save
  end
end