Here's a sketch of my ideal gem for designing, auditing and cross-browser testing HTML emails in Rails applications. This could be built upon the work of the existing 37signals/mail_view.

mail_preview.rb

# Gemfile

group :development do
  gem 'mail_preview'
end

# config/initializers/mail_preview.rb

MailPreview.configure do |c|
  c.litmus subdomain: "company",
           username: "root",
           password: "123",
           name_prefix: "MyApp"
end

# app/mailers/previews/registration.rb

require 'factory_girl'

mail_preview "RegistrationMailer.welcome", "Sent after a user creates an account" do
  user = FactoryGirl.create(:user)
  RegistrationMailer.welcome(user)
end

mail_preview "RegistrationMailer.goodbye", "Sent after a user deletes their account" do
  # or using standard fixtures
  user = users(:bob)
  RegistrationMailer.goodbye(user)
end

# http://localhost:3000/mail-previews/email-1/html

# |----------------------------------------|
# |  Group    | Description of email       |
# | *Email 1* |----------------------------|
# |  Email 2  | *HTML* |  Text  |  Litmus  |
# |           |----------------------------|
# |           |                            |
# |           |                            |
# |           |                            |
# |           |                            |
# |           |                            |
# |           |                            |
# |           |                            |
# |----------------------------------------|

# http://localhost:3000/mail-previews/email-1/litmus

# |----------------------------------------|
# |  Group    | Description of email       |
# | *Email 1* |----------------------------|
# |  Email 2  |  HTML  |  Text  | *Litmus* |
# |           |----------------------------|
# |           |                            |
# |           |  Never tested.             |
# |           |                            |
# |           |  [ Test ]                  |
# |           |                            |
# |           |                            |
# |           |                            |
# |           |                            |
# |           |                            |
# |----------------------------------------|

# http://localhost:3000/mail-previews/email-1/litmus

# |----------------------------------------|
# |  Group    | Description of email       |
# | *Email 1* |----------------------------|
# |  Email 2  |  HTML  |  Text  | *Litmus* |
# |           |----------------------------|
# |           |                            |
# |           |  Last test 14 days ago:    |
# |           |                            |
# |           |  |----|   |----|   |----|  |
# |           |  |    |   |    |   |    |  |
# |           |  ------   |----|   |----|  |
# |           |  iPhone   Android  Blackb  |
# |           |                            |
# |           |  [ Retest ]                |
# |           |                            |
# |           |  Previous tests:           |
# |           |    * 18 days ago           |
# |           |    * 24 days ago           |
# |           |                            |
# |----------------------------------------|