Gimp all the way for general stuff and post-processing. I also used Inkscape sometimes, for more precision work like logos etc.
Recently I've had a go at Art Rage on my windows tablet - tricky to get used to (it mimics oil painting) but got some good and fast results. You can check what it looks like on my game page.
For pixel art, everything is done with Pixartu on my iPad, my all time favourite app! That means I can work on the train to and back from work, not a minute of spare time is lost ;)
I wrote a devblog on tools/software I use a while back, feel free to it check out for more details on what I use to game dev!