HTMLEmail.io - Responsive HTML email templates for startups & developers
TemplateMo - TemplateMo is a great place to get bootstrap templates from your website, which are all HTML5 supported with CSS layout.
Templates by Email Monster - Hundreds of free, responsive email templates. Pick and edit.
Ethereal by HTML5 UP - A fully responsive side-scrolling one-page HTML5 template
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.
UIdeck - Landing Page HTML Templates and UI Kits