Free Online Photo Editor - FotoFlexer
One of the best online photo editors I've ever seen is FotoFlexer. The best thing about it is that you don't have to join or sign up - you can just go to the site and upload a photo and start editing.
The other great thing is the number of tools they provide from effects to frames to curves (an advanced photo editing tool that allows you to adjust the tones, brightness and darkness of a photo).
Click here to use the free online photo editor from FotoFlexer.