Though it's not intended, you can crop photos within iPhone's Photo viewer. The limitation is that images are restricted to screen size of 420 x 320, either in portrait or landscape. The secret is the hidden screenshot app.
- Go to Home/Photos and choose the photo you want to crop.
- Unpinch to enlarge the photo to the magnification you'd like to crop to.
- Tap the screen to hide the controls.
- Hold the Home and Sleep buttons at the same time, then let go. You'll see a white flash when the screen shot takes.
- Tap the screen to show the controls.
- Tap iPhone Libray/Photo Albums/Camera Roll.
