Templates by Email Monster - Hundreds of free, responsive email templates. Pick and edit.
Inovatik HTML Templates - Download 24 FREE HTML templates to help you create beautiful websites and landing pages faster.
MJML - The open source framework for responsive emails
Paperplane - High quality HTML to PDF conversion API for developers.
Stripo - Stripo is an all-in-one email design platform. We enable our clients to build emails of any complexity, including interactive AMP emails, really fast and easy.
Wicked Templates - Responsive HTML templates ready to customize out of the box.