unobtrusive-target-show-hide.js

//gather all the tooltips into an array and hide them
var tips = $$('.tooltip').invoke('hide');
//gather all the triggers into another array, and loop over them
var triggers = $$('.trigger').each(function(elm, idx){
  //link each tip to its trigger, using its position in the source order as the key
  elm.tip = tips[idx];
  //watch each trigger for a mouseover
  elm.observe('mouseenter', function(evt){
    //remove the "hover" style from all triggers
    triggers.invoke('removeClassName','hover');
    //add it to this one that you're hovering over
    this.addClassName('hover');
    //hide all the tips
    tips.invoke('hide');
    //show the one related to this trigger
    this.tip.show();
    //now do the same thing in reverse for a mouseout
  }).observe('mouseleave', function(evt){
    //hide all the tips
    tips.invoke('hide');
    //remove the hover style from this one
    this.removeClassName('hover');
  });
});