What are some alternatives?
Balsamiq - Balsamiq. Rapid, effective and fun wireframing software.
draw.io - Online diagramming application
Moqups - The most stunning HTML5 app for creating resolution-independent SVG mockups, wireframes & interactive prototypes for your next project
OmniGraffle - OmniGraffle is for creating precise graphics like website wireframes, an electrical system designs, or mapping out software class.
Invision - Prototyping and collaboration for design teams
SmartDraw - Instead of starting with a blank screen, SmartDraw gives you the exact template you need.