Mockup Generator - Create attractive product screenshots with just a few clicks
HTMLEmail.io - Responsive HTML email templates for startups & developers
Simple Mockups - Minimalistic devices mockup pack for Sketch & Photoshop
Templates by Email Monster - Hundreds of free, responsive email templates. Pick and edit.
Mokup Frames - Create amazing Dribbble-ready GIFs or videos in a few clicks
MJML - The open source framework for responsive emails