Think flashmob for the internet age. Last year around 300 people came together out of nowhere, checking into simultaneously to the Jewel Bar in Picadilly Circus. Admittedly, that had some planning where as this year we're winging it a bit but hey.
Love Foursquare and the random tips it gives you for your local restaurant, pub or pretty much anything else? Then come together on 04/16 (16/04 on this side of the pond) and join other Foursquare users in celebrating the app. Nothing on offer other than the self-gratification of confusing tourists who wonder why a large group of people have suddenly gathered in one place, and possibly one of those swarm badge things.