Compatible: iOS 4.2
The standard to live by is readily available to majority of iOS devices. It balances a clean interface with accessibility quite well. For those who are intermediate to basic Photoshop features will be right at home with cropping, contrast, brightness, sharpen, soft focus, exposure adjustments as well as several effects being available. This is my go to App for photo effects and highly recommended. If you're looking for more (or if it's too much) you'll be able to safely gauge by sampling with Photoshop Express.
Compatible: iOS 3.0
Easier than Photoshop Express but lacking in specific features such as cropping and manual adjust. Adds effects to pictures to make them more interesting. It mostly gives the impression that your photos are more artsy and is good for a laugh. Readily available to upload to Flickr, Facebook, Twitter or to e-mail or save/print, it's easy to get into but can be limiting for those looking for a specific look.
Compatible: iOS 4.0
Fans of Autodesk will love their Pixlr-o-matic app which fuses the two aforementioned into a streamlined experience with an amazing interface that I fall in love with every time I use the damn thing. Sharing options aren't limited to save to the Photo Roll with access to Flickr, Dropbox, Facebook, Twitter and a few more. There are literally dozens of effects and borders to choose from to give a unique and compelling look to an otherwise boring picture. It lacks the specific sliders and cropping that Photoshop Express offers but Pixlr-o-matic will deliver you right to the end rather than making constant adjustments and wondering 'Is that good?' There's even a randomizer button to throw effects onto a picture if you don't know what you're looking for. It's all good.
Left: Before - Right: After