Gimp will do you fer a spell, but it ain't the most smooth of experiences and is more of a pain to use then it has any fuckin right to be. Sai is a very specialized set of options - anything you can do in there you can replicate in photoshop, but it's a gigantically brilliant option for those who just want to draw.
Finally yer got yerself some Photoshops. CS5 crashes often and takes down my existing work with it. It's a pain to deal with and it takes a few hours to filter out most of the bullshit and tailor the workspace to what you actually need it to do. Somehow it still manages to be user friendly and if you're willing to spend a few hours learning the ins and outs of it, it'll reward you fer the effort and then some.
Course, art programs are a dime a dozen so I'd fully fucking recommend trying out at least a few of them, because sometimes a different perspective is all that's needed to glean some insight into the clusterfuck of art programs out there.