OK, so technically, we are featuring Aviary Advanced, but the company as a whole is a great resource for creative folks.
Originally, Aviary was a suite of tools, including image editing, audio editing and music creation available for free online, but most recently they have started to focus their energy on a simple image editor for web applications.
Fortunately all of their creative editing tools are still available at http://advanced.aviary.com/. Check out what is available, and hopefully they have something you can use on your next project.