There's a free tiny incoming jewell : My paint mypaint.intilinux.com
...its stroke settings control is amazing, rarely so many settings for a ink stroke in other softwares.
Art Rage www.artrage.com
will provide a very natural feeling of traditional brushes, etc, but I see better SketchBook Pro, for ink drawing (or even MyPaint)
Open Canvas is pretty nice too.
I would recommend any of these 3. It depends also on which are your plans with the software.
You have also getpaint.net , if you need some basic image editing operation to complete the workflow and don't want to use Photoshop.
I do use it, Photoshop, a lot, at work...
For inking I prefer the others.
Edit: BTW, Painter is very good, too.