This is a running thread idea created by Highlander. The first one was a Flag Continuation, in which one starts with a blank flag and then each poster has to add another element.
Far more successful and proliferative has been the Map Continuation, which started with North America but has now filled an entire world with detailed countries, each poster carving a new country out of the blank and later adding a flag, history, etc.
Link to “World Gone Mad” Map Continuation Wiki: http://ahmapcon.wetpaint.com/?t=anon