Viewflux - Feedback, prototyping and collaboration for designers
Invision - Prototyping and collaboration for design teams
Savahapp - The all-in-one design sharing, collaboration, workflow and app prototyping platform
Marvel - Turn sketches, mockups and designs into web, iPhone, iOS, Android and Apple Watch app prototypes.
Red Pen - Red Pen lets you upload your design, share a short URL, and get live, annotated feedback super-fast.
UXpin - Design is really about solving problems. UXPin is the UX Design Platform that gets that right.