MJML - The open source framework for responsive emails
HTMLEmail.io - Responsive HTML email templates for startups & developers
BeeFree - BeeFree is an all-in-one solution that works anywhere you are.
Templates by Email Monster - Hundreds of free, responsive email templates. Pick and edit.
Elite Email - Drag & drop email builder for awesome newsletters
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.