iOS: Photo retouching on iOS is always a bit of a pain, and while apps like Pixelmator do an admirable job, Photoshop Fix does much of the same work, but it does for free.
Photoshop Fix is all about making quick, minor edits to photos. As you'd expect, you get tools to adjust levels, fix red eye, and do simple crops. More importantly, you also get restoration and retouching tools like the healing brush, liquify, and the clone stamp. With healing, you can easily remove unwanted elements from images, liquify helps you do minor tweaks to portraits, and the clone stamp allows you to clone an element in an image as often as you like. As far as mobile image editing goes, this is plenty to quickly get an image ready when you're on the go, though don't expect desktop-quality edits. The app is free, but you'll need a free Adobe ID to use it.
Adobe Fix (Free) [iTunes App Store]