Targeted.io - Test your email designs across 30+ clients before you send'm
Really Good Emails - A large collection of good product email design.
Email Love - Email design inspiration, templates and discovery
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.
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.
Templates by Email Monster - Hundreds of free, responsive email templates. Pick and edit.