Every now and then I stage small web experiments. This is one of them.

Check Republic is simple; it's a website filled with check boxes that the visitor can check and uncheck as he or she pleases.
Interaction arises from two features, one is obvious: everyone sees the same checkboxes, if Anna check the 3rd box in the 20th row, Bob instantly sees that box toggled on his screen, too. The other feature is that cursor positions are broadcasted between clients (via web sockets and p2p) resulting in seeing the cursors of other visitors as they navigate the page on the computers - in real time.