browserling - Live interactive cross-browser testing from your browser.
Maze - Beautiful & actionable analytics for InVision prototypes
Sauce Labs - Test mobile or web apps instantly across 700+ browser/OS/device platform combinations - without infrastructure setup.
ProtoPie - ProtoPie is the easiest tool used to turn your UI/UX design ideas into highly interactive prototypes for mobile, desktop, web, all the way to IoT.
Browsershots - Browsershots makes screenshots of your web design in different browsers.
W3C Mobile Checker - Make websites work better on mobile devices