Invision - Prototyping and collaboration for design teams
Responsive Viewer - View multiple screen dimensions in one view
Marvel - Turn sketches, mockups and designs into web, iPhone, iOS, Android and Apple Watch app prototypes.
Responsive Website Test - Test any website on desktop, mobile, and tablet
UXpin - Design is really about solving problems. UXPin is the UX Design Platform that gets that right.
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.