1. GIMP
GIMP (GNU Image Manipulation Program) is a totally free of cost powerful software that is cross-platform i.e. it is available for not only Windows but also Linux and OS X. It is also power packed with a large set of tools and functions which make me admit that GIMP is one of the real competitors of Photoshop.
2. Paint.net
Paint.net is another well known free image editing software that is available only for Windows users.
3. Inkscape
What makes Inkscape enter this top 5 list of Photoshop alternatives is its capacity to deal with vector graphics. Yes Inkscape is capable of implementing vector graphics which makes it stand out compared to the other free alternatives
4. Pixlr Editor (Web based)
Pixlr is one of the few web apps which are strong enough to support basic as well as intermediate editing features.
5. Pixia
Pixia is a Windows only software that has originated in Japan. Pixia is the English version of a Japanese image editing software. Once again, Pixia isn’t all advanced, its a simple tool with a lot of basic editing features which are pretty cool for a free tool.