UserTesting.com - Usability testing has never been easier. Get videos of real people speaking their thoughts as they use websites, mobile apps, prototypes and more!
Maze - Beautiful & actionable analytics for InVision prototypes
Userbrain - Easy, fast, and affordable user testing for websites and prototypes.
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.
UserBob - Provides affordable remote user testing.
W3C Mobile Checker - Make websites work better on mobile devices