I regularly listen to This Week in Startups which is produced by Jason Calacanis, he does a great weekly video which runs for an hour and has in depth interview with VCs and startup founders, who he manages to get to reveal some pretty interesting stories, often gets them to admin to things they may not have before, its one of the better webcasts for startups and a great laugh at the same time.
Jason does the Web equivalent of a live read for his sponsors (those of you who have ever advertised on radio will be familiar with the concept) and one of the companies he mentioned the other day was Invision a new prototyping, collaboration and workflow platform.
Having tried about 10 prototyping/wireframing/mockup apps in the last year for one of the projects I am working on, I can tell you they pretty well all suck big time on the collaboration aspect.
Also commenting and keeping track of problems is also a major headache, invariably you need to switch out to another platform to track the requests/bugs etc and don’t even talk about keeping version control over design suggestions and mockups, its not trivial or commonly done.
So Invision seems to solve the problem for design collaboration and prototyping in a similar way to how Github solves the problem collaboration problem for code and version control.
There is a 30 day free trial and the plans seem reasonable including a free plan for only one project. Check it out here Invision App.