What are some alternatives?
Invision - Prototyping and collaboration for design teams
Atomic - The fastest way to design beautiful interactions
Axure RP - The prototyping and specification software used by 87% of the Fortune 100. Beyond mockups, design interactive prototypes for business solutions, websites, and applications.
Concept.ly - Concept.ly - Free web & mobile(iOS,Android) prototyping and ui mockup tool
UXpin - Design is really about solving problems. UXPin is the UX Design Platform that gets that right.
Pastel - Sticky note-based feedback collection tool for live websites