If you are responsible for creating email campaigns, you might find some of the following links useful:
Litmus lets you preview your campaigns across 40+ real email clients
and devices in minutes. You can now deliver your e-blast with piece of mind without worrying how it will look on the different email clients or web browsers. Litmus is not a free service, but it is worth the purchase if you are handling a large amount of email campaigns.
Campaign Monitor CSS
A complete breakdown of the CSS support for every popular mobile, web and desktop email client on the planet.
Premailer is a quick way of converting your CSS styles into inline style attributes, converting relative links into absolute links and creating text versions of your eblast.
CSS inline is a quick way or converting your CSS styles into inline style attributes.
Zurb Studios Responsive Email Templates
Zurb has created a bunch of responsive email templates… unfortunately they are not supported on Outlook 2007, 2010, 2013 so use at your own risk.
Tuts+ has created a fine example of how to create responsive email campaigns with support for Outlook 2007, 2010, 2013.