Fdclock is a clock to show off many of Cairo's features: it features scalable widgets (including a scalable freedesktop.org logo as the background), and transparency. When used with an X server that supports the Composite extension, you will be able to see proper transparency for the clock, and note how the hands/dots/etc cast darker individual shadows than the rest.

This package comes from the freedesktop.org git module: git://anongit.freedesktop.org/git/xorg/app/fdclock The module name is fdclock.