Really Good Emails - A large collection of good product email design.
Stamplia - High converting and beautifully designed email templates
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.
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.
Email Love - Email design inspiration, templates and discovery
HTMLEmail.io - Responsive HTML email templates for startups & developers