Besides mails_viewer, there are several more choices such as mail_view, letter_opener, mailcatcher and mailsafe. Checkout our blog http://blog.pragmatic.ly/announcing-mailsviewer-a-rails-engi... for more info. There is a link in the github page but I think it's not clear, :)
Thanks for the list. I searched for these months ago and couldn't find much. I ended up using https://github.com/wireframe/email_preview, but some of these look pretty nice.