A few years ago, there was not a lot of good solutions for helping teams prototype user interfaces in an interactive method. Visio and PowerPoint were pretty much king in this domain.
In the last year, i got back into the subject for the different projects i was working on and was delighted with the new offerings.
Personally, i chose MockPlus. It seemed right and did the right thing for me.
If you want to read a more thorough review, follow this link : http://www.cooper.com/prototyping-tools