A distributed object-manipulation client. It's a little like collaborative editors, only instead of editing text, you move around various icons and images. Of course, all the users see the movement in real time, and a colored frame indicates who moves what.
There are many possibilities:
Of course you'd need a client. Ideally, the client would be written in JavaScript, using AJAX and whatnot, embeded on a web page, so that you don't need to install anything and everyone can just go to the page and use it. Less perfect solution would involve a separate client application, or a plugin to one of popular instant messaging clients.