Paperplane - High quality HTML to PDF conversion API for developers.
HTMLEmail.io - Responsive HTML email templates for startups & developers
Startup Emails - Responsive HTML email templates for startups
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.
Really Good Emails - A large collection of good product email design.
Templates by EmailOctopus - Free (do whatever you want) responsive email templates