iOS: The iPad version of Pixelmator released last year was fantastic. Today the app gets an update that brings along iPhone compatibility and a new distort tool.
The smaller screen real estate on an iPhone makes Pixelmator a bit harder to use than on an iPad, but it's still just as powerful. Since the screen's smaller, the app uses a variety of popover panels to select which tools to use, which means you'll need to make a few taps through the contextual menus to set up your tools. It's a bit obnoxious to tap through, but still does a pretty admirable job of making editing photos on your iPhone plausible if not enjoyable.
All the serious editing tricks in the iPad version are here (alongside a new distort tool), so if you have the patience deal with the small screen, Pixelmator on the iPhone is powerful. Either way, if you already have the iPad app, it's worth taking a look at the iPhone since it's free in this update.
Pixelmator ($6.49) [iTunes App Store]