Sortd - Rated the #1 App for Gmail
HTMLEmail.io - Responsive HTML email templates for startups & developers
KanbanMail - A Kanban board for your emails.
Templates by Email Monster - Hundreds of free, responsive email templates. Pick and edit.
Trello - Infinitely flexible. Incredibly easy to use. Great mobile apps. It's free. Trello keeps track of everything, from the big picture to the minute details.
MJML - The open source framework for responsive emails