Nik Software collection is pretty good, I have used it from time to time, and while i would never have suggested buying it, it is free now, so what the hell. You can download it here! :)
It is good to play around with and to see what kind of effects you like with your images, and can give you some instant filters/fixes if you want.
I do not think that this replaces completely processing your images yourself, but at worst it is a bit of free fun! :)