HTML5 uploader has a built-in cropping tool. Each photo added to the upload list may be opened using an image editor. When you turn on the image editor, an appropriate button appears on the toolbar which appears when you hover a mouse over a thumbnail in the upload list. In the editor, a user may select a portion of image and "cut" it. Through the uploader settings, you may control whether you want to allow a user to crop with an arbitrary aspect ratio or force it to have specific proportions (like 2x3 or 3x4), allow only album or portrait orientation or it may be changed, etc.
Note, the crop is not "destructive". A user may always revert to the original state. You can always access crop boundaries through the JavaScript. Also, you may configure the uploader to apply crop only on particular file copies (e.g. upload a cropped preview image and an original file along with the crop boundaries so that you could allow a user to re-crop the image later on).
Apps\ImageCrop folder of the Upload Suite 8 SDK.