The way I see it there are 2 classifications. Wireframing tools and then mockup feedback tools. Wireframing * Mockingbird (http://gomockingbird.com http://gom

The way I see it there are 2 classifications. Wireframing tools and then mockup feedback tools. Wireframing * Mockingbird (http://gomockingbird.com http://gomockingbird.com) - Built on Cappuccino, best designed of the bunch * Napkee (http://www.napkee.com/ http://www.napkee.com/) - Flex * Hot Gloo (http://www.hotgloo.com/ http://www.hotgloo.com/) - Just a web app I believe * Pencil Project (http://www.evolus.vn/Pencil/ http://www.evolus.vn/Pencil/) - Firefox add-on * Several more wireframing type apps... Mockup feedback * Mocksup (http://mocksup.com http://mocksup.com) - Make your mockups clickable (Full disclosure, I'm co-owner) * Notable (http://www.notableapp.com/ http://www.notableapp.com/) - Mark up websites and mockups with comments * ConceptShare (http://conceptshare.com/ http://conceptshare.com/) - Same * Several others that focus on just mockup feedback, not much differentiation