What are some alternatives?
Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.
Invision - Prototyping and collaboration for design teams
Moqups - The most stunning HTML5 app for creating resolution-independent SVG mockups, wireframes & interactive prototypes for your next project
Zeplin - Collaboration app for UI designers & frontend developers
Webflow - Build dynamic, responsive websites in your browser. Launch with a click. Or export your squeaky-clean code to host wherever you'd like. Discover the professional website builder made for designers.
ProtoPie - Compose elaborate interactions and animations easy, and build sensor-aided prototypes without a line of code.