Templates by Email Monster - Hundreds of free, responsive email templates. Pick and edit.
Dragon Responsive Email Designer - Dragon Email Designer provides an intuitive drag and drop newsletter design tool.
MJML - The open source framework for responsive emails
Marble Responsive Email Designer - Marble helps you design beautiful and mobile-friendly emails and newsletters.
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.
Kingmailer - Kingmailer helps you send, receive and route/forward emails.