Personal computing applications are constantly increasing their potential power, thanks to steadily growing hardware capabilities and large diffusion of high-quality multimedia output devices. At the same time, mobile communication tools are becoming an integral part of our everyday life, with new advanced functionalities offered at an unrestrainable pace. Although the way we interact with information machines is substantially the same since 20 years--based on keyboard, mouse and window metaphor--other communication modalities are possible, and shortly may become popular as additional interaction methods. Given the paramount importance of the "interface" in present computer applications, no alternative should be ignored, as it could greatly...