Ambient Design/ArtRage
From Mobile PC Wiki
Free Version "ArtRage 2.11 Free Released: ArtRage 2.11 Free has been released in both English and French. You can download this version for Windows or OS X from the Download page." (on the ArtRage site)
Slightly earlier version "Ink Art" is included in the Microsoft Experience Pack for Tablet PC
Also full version Approx $20 USD.
See Chad Essley's excellent comparison Artrage 2.0 / Alias Sketchbook Pro
