GIMP is a open source paint program that has some features comparable to Photoshop - considered the open source Photoshop. Open source means it is free to download and develop further - there is a nice community of folks continuing to make improvements to it.

I tried it a few times but just couldn't get used to the UI to be honest.. perhaps it has improved now but since I'm fortunate to have access to Photoshop CS3, I just stick with that.