HTMLEmail.io - Responsive HTML email templates for startups & developers
Mashup Template - Collection of beautiful free HTML templates
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
Inovatik HTML Templates - Download 24 FREE HTML templates to help you create beautiful websites and landing pages faster.
Paperplane - High quality HTML to PDF conversion API for developers.