YouClap - Challenge and be challenged by friends and brands.
Really Good UX - A library of screenshots for UX inspiration
Spinnner - Challenge your friends to a spin-off (literally)
Startup Timelines - How the frontpages of famous startups changed over time
UXTesting - Discover how your mobile users feel
Marvel - Turn sketches, mockups and designs into web, iPhone, iOS, Android and Apple Watch app prototypes.