Citrify, iPiccy, PicMonkey, Ribbet, Picadilo, Fotor, FotoFlexer or Lunapic are great all round web based and FREE editing tools. Adobe, makers of the expensive Photoshop, has a free website version in Photoshop Express. If you are a former Picnik user (Picnik closed in 2012) than Picmonkey and Ribbet will be your best choices.
If you need/want more features downloading GIMP is a great options. Still free, but you have to download the software.
http://www.edit-photos.net
http://www.squidoo.com/How-to-Picnik
http://www.squidoo.com/picadilo-photo-editor
http://www.squidoo.com/fotor-photo-editor
http://www.squidoo.com/ribbet-photo-editor