This is an example of what the Internet can do. If you have a good idea, you can assume that a bunch of other people have the same idea. It's the team that executes quickly, effectively, and with some luck that will succeed. I wanted to do some mobile prototyping and ran across the following list:
I am interested in HTML5 app dev so am looking for multiple platform support. Many are iPhone/iPad only. There are a set of solutions that are designed to string pages of graphics together adding hotspots to create a prototype and others that let you mock up prototypes using components. I'm interested in the latter since I'm heading towards creating a mobile app, not just designing one. Some that I've found and am experimenting with:
http://proto.io -- This is one that I used to create a simple 5 screen prototype of an attendance app for work and am favoring now because I like the feel of the application. The Plus plan is $24/month as of this post.
http://www.justinmind.com -- This one is a Windows/Mac app with web tie-in. It looks like a more comprehensive and full featured product for a professional designer.
Update: found this site recently...