Linux
Picnik or lunapic are great free web based (nothing to download) editing tools. If you need/want more features downloading GIMP is a great options. There are also free tools to add text blurbs, to make collages, to put your photo into other images. There are a number of tools that allow you to put the image of a face into other photos for some fun creations. Also, you could try Paint net.
Of these two downloads, I like GIMP better, but it's really Linux oriented (shares Linux UI styling), not that it's a bad thing. however.... I do have Both of them installed, just cause Paint net has filters that GIMP does not have.
