Due to recent increase of computer power and decrease of camera cost, it became very common to see a camera on top of a computer monitor. This paper presents the vision-based technology which allows one in such a setup to significantly enhance the perceptual power of the computer. The described techniques for tracking a face using a convex-shape nose feature as well as for face-tracking with two off-the-shelf cameras allow one to track faces robustly and precisely in both 2D and 3D with low resolution cameras. Supplemented by the mechanism for detecting multiple eye blinks, this technology provides a complete solution for building intelligent hands-free input devices.The theory behind the technology is presented. The results from running several perceptual user interfaces built with this technology are shown.