Do you own Copic markers? Do you need a simple, fast and easy to use sketching tool, which simulate in a realistic way the various marker tips, color wetness and offers the Copic color palette? Autodesk released a commercial full featured and and a limited and free Sketchbook Copic edition. The interface is very clean, non obtrusive, the drawing engine is fast. While the free edition only offers a basic set of colors and rudimentary drawing tools - this resembles also only what I would use in real life such as marker paper, markers and a straight edge.
The software works on OS X and Windows.