Ethereal by HTML5 UP - A fully responsive side-scrolling one-page HTML5 template
Templates by Email Monster - Hundreds of free, responsive email templates. Pick and edit.
Mashup Template - Collection of beautiful free HTML 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.
Inovatik HTML Templates - Download 24 FREE HTML templates to help you create beautiful websites and landing pages faster.
MJML - The open source framework for responsive emails