Ranked in these QuestionsQuestion Ranking
Pro Supports web and mobile prototyping
Allows creation of interactive HTML mock-ups which is accessible for both websites and mobile applications.
Pro Lots of options for sharing your work
Great web options, but you can also save your wire frames out to Word and convert the notes to annotations.
Pro Resuable components
You can create a component (widget) that you can use over and over again.
Available for use on Windows and Mac.
Pro Collaboration tools
You can set up a project with SVN-like check-in/out system to work with others.
Pro Offers resources to help you get proficient with the software
There is a wealth of online training including videos for getting the basics down, a great community to mine, and support team to help assist with more technical details.
Con Moderate learning curve
Con Expensive [$289-$589]
There is a free trial to work with Axure, but there is a hefty price for the standard edition at $289. Even more so, the Pro edition will cost $589.