Templates by Email Monster - Hundreds of free, responsive email templates. Pick and edit.
HTMLEmail.io - Responsive HTML email templates for startups & developers
Paperplane - High quality HTML to PDF conversion API for developers.
MJML - The open source framework for responsive emails
Really Good Emails - A large collection of good product email design.
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.