Last.fm Disco!

Go on last.fm and paste the following into the address bar:

javascript:function toggle(){
t=setTimeout("toggle();return false", 300);
LFM.Display.colourToggle(this, LFM.String.toRedStr, LFM.String.toBlackStr); return false;};
toggle();
return false;

and hit return!

(However, It seems to go a bit weird if you’ve got multiple browser windows open..)