What are some alternatives?
Atomic - The fastest way to design beautiful interactions
Invision - Prototyping and collaboration for design teams
RedMark - Redmark helps graphic designers work better and faster.
Concept.ly - Concept.ly - Free web & mobile(iOS,Android) prototyping and ui mockup tool
Pixelapse - Visual version control and collaboration workflow for open source and private design projects.
Marvel - Turn sketches, mockups and designs into web, iPhone, iOS, Android and Apple Watch app prototypes.