HTML E-Mail Boilerplate
This website and its sample code creates a template of sorts, absent of design or layout, that will help you avoid some of the major rendering problems with the most common email clients out there — Gmail, Outlook, Yahoo Mail, etc. While not plug and play (you know, you'll have to do some work ;-), it will provide some helpful examples and snippets that will keep your email design rendering as true-to-form as possible.
Duster
… an Automattic-produced theme designed and developed by the Theme Team which, apart from the custom possibilities you’re acquainted with in most of our themes — features like custom backgrounds, headers, and menus — is also equipped with a custom Showcase Page Template that can propel your site up to a new level.