What are some alternatives?
Atomic - The fastest way to design beautiful interactions
Marvel - Turn sketches, mockups and designs into web, iPhone, iOS, Android and Apple Watch app prototypes.
RedMark - Redmark helps graphic designers work better and faster.
Invision - Prototyping and collaboration for design teams
MockFlow - MockFlow is an online wireframe tool for software and websites
Pixelapse - Visual version control and collaboration workflow for open source and private design projects.