Super simple offline sketchbook app for the web in the age of touchscreens and tablets. It's your digital moleskine.
iPhone/iPod touch version
You might also want to get a capacitive stylus from Amazon.com.
SketchThing is made by Chris McCormick and is Free Software (LGPL). Get the source code, or clone it with
bzr branch http://mccormick.cx/projects/SketchThing/.