Pixlr is a really good online editor. It's not as powerful as GIMP or Photoshop, but it's browser based, has all the tools you'd expect and it's free. Really a cool tool to try out.
Aviary is another good browser based tool and they're expanding it to other platforms. It's more powerful then Pixlr, but I find that if I do anything big my browser can't handle it and crashes. So for big projects I crack open GIMP, for small ones I stick with Pixlr.