Firstly to get any decent results with post processing you really need to shoot in RAW as this will allow you to push the image in whatever direction you want (within reason). If you shoot in Jpeg or Tif then you are very limited in post processing and can generally brighten an image and crop it, but there is not much else you can do in terms of exposure or colour enhancement.
If you shoot Nikon there is a free programme called NXD and it is very good. Basic but covers most of my needs.
The latest iphones have some fantastic image processing built in. Soon point and shoot cameras will be a thing of the past as the cameras in phones are getting that good.
Let me know how you go.
