What are some alternatives?
LucidChart - LucidChart is the missing link in online productivity suites. LucidChart allows users to create, collaborate on, and publish attractive flowcharts and other diagrams from a web browser.
Invision - Prototyping and collaboration for design teams
SmartDraw - Instead of starting with a blank screen, SmartDraw gives you the exact template you need.
Proto.io - Create fully-interactive high-fidelity prototypes in minutes that look and work exactly like your app should. Sign up for a free 15-day trial and get started.
iRise - iRise combines fast prototyping with integrated requirements management to ensure you're managing the right requirements and user stories.
Flinto - The prototyping tool designers have been waiting for.