Really Good Emails - A large collection of good product email design.
MJML - The open source framework for responsive emails
Email Love - Email design inspiration, templates and discovery
Email Design System for Sketch - Sketch file with 10 email templates for designers
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.
HTMLEmail.io - Responsive HTML email templates for startups & developers