Responsive Viewer - View multiple screen dimensions in one view
Anima App - Design, get feedback, convert to code, publish, iterate.
Responsinator - Designed to test responsive websites on different devices.
Sketch Repo - Collection of resources for anyone who uses Sketch
LT Browser by Lambdatest - LT Browser is a free dev friendly browser for developers and testers. You can see mobile view of website on different screen sizes and resolution on 50+ pre installed view ports. You can build, test and debug your website faster then ever.
Git Sketch Plugin - Version control for designers