So I've just started to realize the witchcraft that is Paint Tool SAI. There's a "portable edition" if you don't like trial versions.
What's so great about it? Lines! Smooth, clean, anti-aliased lines! If you hate squiggly, jittery lines because of your stupid, uncoordinated hands, use this program. It's almost like the brush tool in Flash, but better. On top of that, its easier to use and isn't as confusing or complex as Photoshop. You can adjust how much the software corrects your lines and is much lighter on performance for your PC.
Don't have a tablet? You can use a mouse! Instead of hand-drawn you can use vector layers for your line work and morph your lines with anchor points. And you can adjust your line width all along the lines with vector mode.
