NVIDIA released another demo today which renders a very realistic human head. You can download it over here at NZone.
The minimum requirements to run this demo smoothly are pretty high. According to NVIDIA you need their latest GeForce 8800 Ultra graphics card to run this demo, other cards will run it too but with less frames per second. Other requirements include Windows XP or higher, ForceWare 97.44 drivers or higher and 512MB memory.