The following technologies can help support the sketching process in different ways. However, take care that they fit your style – remember that a pencil is cheap, easy to use, and always with you.
Livescribe Smartpen. The Smartpen digitally records your sketches as well as the talk that you speak while drawing. You can then replay that as a ‘pencast’. Play the video on the website to get an idea of how it works.
Balsamiq Mockups is a drag and drop interface builder that let you sketch out interfaces quickly. Interface widgets have a sketch-like appearance.
Bluebeam PDF Revu is a PDF drawing editor which is quite nice to use on a pen-based computer. It lets you save sketch elements as a library for later reuse, and export vector graphics to other software (e.g., Illustrator).