Stamplia - High converting and beautifully designed email templates
Targeted.io - Test your email designs across 30+ clients before you send'm
Really Good Emails - A large collection of good product email design.
Pistachio Email Templates - Delicious newsletter templates built for automation
Passport by Mailjet - Create beautiful, responsive e-mails
Mail Designer 365 - Discover Mail Designer 365, the easiest way to design mobile-optimized HTML emails, templates and newsletter designs. Your HTML email designer for Mac.