Autodesk Sketchbook Mobile Express (SBMX) and I can't stop playing with it! It seems pretty fine-tuned for both amateur sketchers and serious graphic artists alike. If you are in the latter category, you may consider getting the full version which includes more tools. However, SBMX has plenty to offer without upgrading.
The interface is intuitive and easy to use. You have a variety of tools to work with (pencil, pen, marker, various paint brushes, shading) and you can adjust the size of the brush and the opacity of the color. You can also choose any color from a color-wheel. Possibly the best part is that you can also work with various layers. When you are finished, you can obviously save your work and share it with the world.
Give it a go!