Topol.io - Create professional Email templates easily - Drag & Drop Email Builder.
Dragon Responsive Email Designer - Dragon Email Designer provides an intuitive drag and drop newsletter design tool.
BeeFree.io - Bee is MailUp's drag-&-drop editor for responsive design emails.
Marble Responsive Email Designer - Marble helps you design beautiful and mobile-friendly emails and newsletters.
MJML - The open source framework for responsive emails
Kingmailer - Kingmailer helps you send, receive and route/forward emails.