Install

Rails 3

Include in the projects Gemfile

group :development do
  gem 'growl_mailer'
end

Then in the development environment set ActionMailers delivery method to :growl

config.action_mailer.delivery_method = :growl

Rails 2

I need to test on a rails 2 ;)

Options

There are lots of options and tricks to play with but I want to wrap this experiment in testing before fully documenting.