If you enjoy photography, iPhoto for iOS could be the best $4.99 you've ever spent on an iPhone or iPad app.
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Ever take a great photo marred only by the fact that it looks tilted? iPhoto has a very cool fix for that: the crop and straighten tool. Tap the dial to activate your iPhone or iPad's gyroscope; now physically tilt your iPhone or iPad device this way and that until the picture straightens out. The flipped page icon (next to the Edit button in the top right) lets you flip between the original photo and the straightened one to check your work.