SAI is tablet driven drawing software. You get pen, pencil, airbrush, paintbrush, etc tools. Everything recognises pen pressure by default. It does colour mixing very well and you have lots of options you can adjust. It supports the typical formats as well as .psd (which is incredibly helpful). And the whole thing is very lightweight as the program is only a few megabytes in size.

And yes, the whole thing is freehand which is why it took me ages (colour adjustments and stuff aside I mean).