GIMP is a very powerful yet often under rated free image editing software. Does most of the stuff the more complex
software does. Free too
Shame it's so complex to use.
I was quite surprised by the latest version of GIMP. I have always been a gimp naysayer but the latest version is much improved. It's coming along nicely.
That said, I wouldn't use it on a daily/weekly/monthly basis and would only use it if I was stuck for Photoshop or Pixelmator but it's definitely worth a consideration.