Among the various photo editors, iPhoto has my favorite implementations for straightening an image. If iPhoto detects an obvious horizon line, it draws a line across it when the Crop & Straighten tool is active; tap one button at the right edge of the line to straighten it (see Figure 5).
Figure 5 iPhoto suggests straightening.
Instead of trying to wrangle a rotating rectangle, or turning the image with two fingers in a circular motion (although you can do the latter if you want), slide the dial at the bottom of the screen left or right to rotate the image in fine increments.
Wait, did I say those were my favorite parts? I'm saving the best one for last because it's cool—and ultimately useless. Yes, you read that correctly. Tap the dial once to highlight it. But instead of dragging your finger to adjust the rotation, turn the entire iPad as if it were a car's steering wheel. Using the iPad's accelerometer and gyroscope, you can adjust a photo's straightness by moving the iPad itself (see Figure 6). It doesn't make sense—sliding the dial is much easier and more accurate, but the very fact that Apple included this gyro mode gives the app a sense of whimsy, which makes it my favorite.
Figure 6 Turn the entire iPad to straighten the photo.