FlipBook offers a suite of outstanding drawing tools. You can paint, erase, color and more. You can import images from your camera roll, sample them with the eye dropper, and even use layers to create complex images. If you're familiar with any kind of Photoshop-style image editing, these tools will make instant sense. And, like Photoshop, the tools all offer customization options. You can enlarge or shrink your drawing elements or change their style using the toolbox.
What makes this amazing is that Anon has done this on an iPhone. With a limited 320-by-480 screen and a limited interaction vocabulary, he's created a powerful user interface that makes sense. It's incredibly easy to use. For example, you can flip your way through (and edit!) your frames using a control that's straight out of the page-picker from Mobile Safari. Or you can resize your eraser with a custom-built slider that shows you exactly what size you'll end up with.
Admittedly, it took me a while to get used to some of the gesture shortcuts. For example, a two-fingered single tap brings up the primary toolbox. Fortunately, mastering these shortcuts isn't a necessity. Anon put in simple buttons that offer direct access to core functionality in addition to the gestures. So if the gestures aren't firmly in your muscle memory, you can just tap your way to the functionality you need.
There's also multiple undo and redo support. You can move back and forth through your history. Single-tap the undo button to go back, double-tap to redo. There are even undo buttons in some of the sliders to return your settings to the exactly point you left them.
Another thing that Anon nailed is application persistence. When you leave the app and come back later, you return exactly where you left off. This makes handling interruptions like taking phone calls a snap. You can stop, leave the program, and know that when you return you can pick up where you were.
FlipBook lets you play your animations back directly in the program using standard QuickTime controls. For now, the sharing options are limited to uploading your QuickTime videos to the flipbook.tv site or saving frames to your photo library, but Anon writes that he's looking into other sharing options.
All in all, FlipBook is a well-designed program with excellent interaction details. FlipBook is available for iPhone and iPod touch and costs $9.99.
Here's a guided tour video from flipbook.tv that walks you through the key features.