Is there anything else that is a) free, b) cross platform, and c) has the powerful features of GIMP? I use GIMP because of those three factors (and to be clear,

Is there anything else that is a) free, b) cross platform, and c) has the powerful features of GIMP? I use GIMP because of those three factors (and to be clear, I would happily purchase Photoshop, but I switch OSes all the time and want something I can use everywhere).