Answer:
There is the GIMPShop, which is the GIMP, with a modified UI, so that it looks and feels like photoshop, if that's what you are used to, or prefer.
There is also an online program called "Splashup", previously known as "fauxto". It is very similar to Photoshop, but it is only available online and cannot be downloaded for offline use.
(Aviary is in the same boat as Splashup)
Corel Paint Shop is a popular alternative to Photoshop, though it has less features than GIMP or Photoshop.
Also, there is a great site where there is a whole collection of photo-creating and editing software and even music editing software that you can use online. You don't have to download anything. It is here: http://aviary.com/