Spark Mail - Spark helps you take your inbox under control. Instantly see what’s important and quickly clean up the rest. Spark for Teams allows you to create, discuss, and share email with your colleagues
HTMLEmail.io - Responsive HTML email templates for startups & developers
Email Testing By Inbox Pirates - Preview, Analyze & Test emails with one click.
Templates by Email Monster - Hundreds of free, responsive email templates. Pick and edit.
iMailair - We all know that email is broken.
MJML - The open source framework for responsive emails