Photoshop Alternative : The GIMP:
Yes, it’s a terrible name - it stands for GNU Image Manipulation Program, and although its main platform is Linux, the GIMP is available for Windows and Mac too. This is as near to a free Adobe Photoshop as you can get. Like Photoshop, the GIMP isn’t aimed at beginners, but despite a rather odd interface, it is very, very good.
I really like the GIMP. All you need is here, so:
if you want a free alternative to Photoshop or Lightroom, this is it.
In fact I find the latest version of the GIMP far nicer to use than recent versions of Paint Shop Pro or Photoshop Elements. The GIMP doesn’t have any wizards to guide you through, but it does have all the basic tools including color correction, levels and curves, layers and sharpening. For serious photographers this offers far more power and control than you would expect in a free package, and it is possible to arrange the interface to look more normal, to me at least.
Installing and Using the GIMP