Preloadr is a simple image editing web application that works with your Flickr account. Using the Flickr API, Preloadr imports your images and lets you apply effects like: sharpening, color correction, blur, auto contrast, granulation or grayscale transformation. You can also rotate the picture, crop it, apply a zoom factor, change opacity, add text. You can create and manage layers, set basic properties, enable or disable them. It's nice that you have undo /redo options at every step and the filters are applied pretty fast. After finishing processing the photo, you can save it as a new image in your Flickr account or replace the original image if you have a Flickr Pro account.
All in all, you won't find advanced Adobe Photoshop options and tools (or Gimp if you want a free Photoshop), but Preloadr is a great way to retouch your photos online.