The cairo-clock from MackSlow indeed has very good look and feel. But it’s transparent background depends on cairo’s alpha channel and Xorg’s composite extension. I did not enable this extension by default because gnome doesn’t have it’s own composite manager but rely on xcompmgr.
But xcompmgr always causes gnome-panel to be overlayed by other window. Then I think, “this clock almost won’t move after it was launched, why not make a fake transparent background for it?” So that’s it, the screenshot: