GIMP has many useful photo editing tools such as the ability to make quick crops, rotation, and color adjustments. It also has more finite controls like curves, hue, saturation, channel mixing, layers, masking, sharpening, clone, heal and levels. It can handle a wide range of file types including JPEG, TIFF, PSD (Photoshops native file format), GIF and more. The program is also highly extendable and has a huge library of freely available plug-ins listed on the site.
http://digital-photography-school.com/gimp-an-introduction#ixzz0Ja9nY9RU&D