Time: XKCD's slo-mo time-lapse comic

On March 25, Randall Munroe ran a strip called Time, an enigmatic, wordless image whose tool-tip was "Wait for it." Ever since, the strip has been updating with subsequent frames, all of them making up a time-lapse animation of a lovely story about a day of sand-castle building at the beach.

The XKCD Wikia entry for the post has animated GIFs and a slideshow showing the progress to date. It's really coming along nicely, and Randall's done some clever things with the back-end to stop people from previewing future frames.