This morning on HN: The Colour Clock (notice the .uk in the domain :).
An elegant clock with changing background color. The exact color is obtained by interpreting the hour, minute, second values as describing the Red, Green, Blue components of the color.
My only tweak on this idea is to make the color-cycle continuous. When the seconds move from 59 to 00 there is a jump in the Blue value from maximum to minimum, which means a not-so-smooth transition in the background color.
Here is the working version:
smooth cycling color clock.