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If you are looking for some inspiration the team at Zurb have created a nice set of Responsive Email Templates , in addition to having some handy tips on that page too .
http://www.zurb.com/playground/responsive-email-templates
Other useful resources
Campaign Monitor Templates http://www.campaignmonitor.com/templates/
More here:
Updated October 10, 2020
MJML Responsive Email Templates → https://mjml.io/templates
Litmus Free, Modern HTML Email Templates → https://litmus.com/community/templates
Premium email templates from Theme Forest → http://themeforest.net/category/marketing/email-templates
Antwort - Responsive layout for emails → https://github.com/InterNations/antwort
Ink - Responsive email framework from Zurb → http://zurb.com/ink/
32 responsive email templates (list post) → http://mashable.com/2013/11/08/email-responsive-design/
Stripo → https://stripo.email/
Sendy Email Builder → https://github.com/coderbag/Sendy-Email-Builder
Postcards → https://designmodo.com/postcards/
Topol.io → https://topol.io
BEE free → https://beefree.io
EDMdesigner → http://edmdesigner.com/
MailStyler → http://www.newslettercreator.com
Campaign Editor → http://designer.moosend.com
CoffeeCup Responsive Email Designer → https://www.coffeecup.com/email-designer/
Mail designer 365 (Mac app) → https://www.maildesigner365.com/
Cheers @Ben more good choices there :)