I haven’t needed an image cropper yet…YET, but if I did, this is a pretty great little piece of DHTML. You could easily tie it into a server call that crops the actual image using GD based on the coordinates generated by the DHTML. If I’m bored one day, I may just tie it into MyScreenshot (**SHAMELESS PLUG**) 
SpeedingRhino » DHTML Image Cropper
This project consists of a Dynamic-HTML interface for cropping images. The user creates a crop selection by dragging a rectangle inside the image and then adjusting it.
