If you're talking about drawing, then SAI Paint Tools is the best one I can think of. You can download a trial of it, but I'm sure if you google hard enough you can find the full version out there.
Photoshop and Corel Painter are also good alternatives for more advanced painting but they're a lot pricier and harder to find 'free' versions that aren't laden with viruses.
But for something completely and legitimately free, try http://mypaint.intilinux.com/
. I don't use it myself because I have SPT, but from what I've heard it comes as a pretty close alternative.