Lean startup methodology strongly emphasizes paper prototypes: What’s the simplest, fastest, lowest-cost thing you could do to increase learning & decrease risk? To that end, AppSeed aims to let you sketch on paper, then turn the results into functioning, HTML-based app prototypes:
Interestingly, it ties into Photoshop:
Test your design on the phone and edit it in Photoshop through PS Connection. This creates a Photoshop document that has all your drawn elements on their own layers, giving you the pixel perfect control to move your design into the next stages of production.