Here's a sweet new Mac app from Facebook to help you design prototypes for your app ideas. You can use this to pitch an idea to possible investors or show a client what type of app you could make for them. It can also take your idea and help you understand what design works and what doesn't. Looks like this is a free app too, so you don't have any excuses. :)
Origami Studio is a free design tool created by Facebook and available for Mac. It allows designers to rapidly build and share interactive interfaces.
Origami Studio from Facebook