Smashing Magazine has a new post today about using jQuery and GD to manipulate images to upload an image, crop it and save that version to the remote server.

One of the numerous advantages brought about by the explosion of jQuery and other JavaScript libraries is the ease with which you can create interactive tools for your site. When combined with server-side technologies such as PHP, this puts a serious amount of power at your finger tips. [...] Sure, there are plugins out there that you can use to do this; but this article aims to show you what’s behind the process. You can download the source files for reference.

They walk you through the little bit of setup you'll need before things will be working and then move right into the upload form and PHP handling that goes with it. The Javascript that goes with it is all home-grown (not a plugin) and cleverly uses a DIV to simulate the bounding box for the crop.