On the surface, scroll clock looks like a simple visual trick: Look, it's a clock made out of moving scroll bars! This is a marginally clever animation! Then you realise that every one of them actually works.
And while it may be a decade late, this is the first truly compelling use for Windows' old Active Desktop feature I've ever seen. This one's for you, inexplicably extant Windows ME users. [Scroll Clock — Thanks, Marco!]