There is an explosion of tools designed to help you do rapid prototyping of software and interaction using the iPad. Some of these are specifically designed to help you prototype iOS apps, but some of them are general prototyping tools. You might also want to check out this handy comparison table.

• iCardSort: This is a great little app to do some simple card sorting using your iPad on the go. Create the deck, and then whereever you go, shuffle the deck, hand it to people, have them sort it, and then email the sorted deck to yourself. Pretty soon you’ll have a whole bunch of sortings that can help you understand the most intuitive groupings for your apps navigation or functionality.
• MockingPad: MockingPad is a simple black-and-white wireframing tool that comes with lots of built-in widgets for web apps, desktop apps and mobile apps. You can populate your widgets with your own text, but you can’t import any images. The biggest drawback to this $10 app is the lack of interactivity – you can’t link between screens or preview the prototype. All you can do is create static screens. I would expect more for $10. Best feature is that your screens can be shared in various formats (HTML, SVG, PDF), through DropBox or email.
• SketchyPad: Like MockingPad, but cheaper ($5), SketchyPad is basically a wireframing tool. It allows you to build colorful wireframes, import images, edit the text of the widgets, and then send your wireframe sketches via email. The coolest feature is that your wireframes can be exported to Balsamiq’s bmml format, so you can sketch your wireframes on the go, and then send the best prototypes on to Balsamiq to refine them further. Not a bad little app for $5, but there is no interactivity here.
• iMockups: This is a decent little interactive wireframing app that gives you a good amount of interactivity for $7. You can import images, make links between pages, add from a decent set of widgets, and then preview your web, desktop or mobile prototype. When you are done, you can export via email to jpg, pdf or Balsamiq bmml formats. This is probably the best low-cost iPad prototyping tool that generates interactive prototypes.
• Adobe Proto: This $10 app is one of Adobe’s Touch Apps, and is designed mainly for wireframing web pages in black and white. You can’t import images or export to anything other than Adobe’s Creative Cloud, which is not very conducive to sharing widely. The interaction is limited to creating links between pages. The coolest feature is the ability to quickly add widgets by sketching gestues on the screen, though it is unclear how many gestures I could/would memorize.
• App Cooker: This is the most expensive of the iPad prototyping apps, at $25 currently, and soon to go up to $50. It is also the most full-featured, and is focused as much on the planning and business case building as it is about prototyping the screens. AppCooker features a calculation screen that helps you work out how to price your app, it features a description screen that helps you describe your app and figure out where it sits in the spectrum of apps. The interactive wireframing support is also really good. Overall, this is a fantastic, but expensive app.
• UISketcher: The propoents of fast and iterative sketching will like this very simple (and inexpensive $4) app. It has a few pens, a highlighter and a whiteout eraser, as well as undo and redo. You can choose to do a one-up single page sketch, or use a 6-up template to sketch 6 different pages together. Type in your annotations on the side and email the sketch to your collaborators or share it via Dropbox.
• Interface HD: Interface HD is a program that allows you to prototype iOS apps only, and exports your , prototypes to XCode, allowing for evoluationary prototyping. This program puts strong restrictions on what you build – for instance you can only add certain widgets to view containers. Thus, it forces you to stick to what is programmable in the end.