The Wiredness online image editor has an incredibly spartan interface. Just a menu bar. No sign up, no fuss, just upload the photo and start tweaking. Nice. Oh and there’s an API too.
The Wiredness system includes a simple API which allows other web applications to pass through the information for an image. This will allow the user to modify that image using the Wiredness system and then save that image back to the calling web application. They can also chose to cancel the modification at any time and it will return them to your site with the original unmodified image.