Invision - Prototyping and collaboration for design teams
UI Recipes - Weekly 15 min lessons on UI design from the hottest apps
Marvel - Turn sketches, mockups and designs into web, iPhone, iOS, Android and Apple Watch app prototypes.
UI Movement - The best UI design inspiration, daily
UXpin - Design is really about solving problems. UXPin is the UX Design Platform that gets that right.
Design Principles - An open source repository of design principles and methods