I don't really like GraphicsGale which was recommended by tehwexxl0rz (eg. it's Windows only), but you know, even MS Paint would be better for learning to pixel than Photoshop. I personally recommend Grafx2 - it has all the nice pixel stuff, an unobstrusive GUI, scripting, customizable keyboard shortcuts, advanced pixel-oriented drawing effects, layering, and excellent palette editing. It only really lacks for animation support.
It seems to always go back to this, doesn't it? I'm just going to say that if you are using Mac, you are basically forced to use Photoshop, because since there are top programs for Mac, you only need one professional one. Even though there is Pixen, I heard it constantly crashes. Gaphics Gale is very suited for pixel art and I reccomend it. Not to offed you, Ai, I am just saying we have different views. If you can''t afford Photoshop, you can use GIMP, it is fairly easy to use. Though, I think you should stick to programs that focus on pixel art. You never told us: what kind of computer are you using?