Google Open Source - All of Googles open source projects under a single umbrella
HTMLEmail.io - Responsive HTML email templates for startups & developers
LaunchKit - Open Source - A popular suite of developer tools, now 100% open source.
Templates by Email Monster - Hundreds of free, responsive email templates. Pick and edit.
Open Source @IFTTT - A collection of IFTTT OSS projects.
MJML - The open source framework for responsive emails