Mocking up a Mac app idea

[MadDoc]

Hi,

I have an idea for a new Mac app that I'd like to work on. I'm hopeless at art and I need to mockup a GUI.

Can anyone suggest some software that I can use to experiment with the layout of this nascent app idea? It would have to come with templates for modern OS X UI controls and, preferably, be under $100.

Oh, if there's something that could do this on the iPad too then all the better (I travel a lot...)

 

[Charlie Sheen]

Xcode.

 

[MadDoc]

I won't be coding the app in Xcode (I'll be using REAL Studio).

 

[Charlie Sheen]

Dous it have a UI builder?

 

[adib]

Keynote template

There is a guy that sells Mac GUI widget templates for $9 - you can use it with Keynote, PowerPoint, or Impress.