Twitter shows additional actions when you hover over a tweet. Here's some jQuery that does the same thing. It assumes the item to show is within the item being hovered over.

Twitter Hover CSS

$('.parentItemToHoverOver').hover(
  function () {
    $(this).find('.childItemToShowOnParentHover').fadeTo(500,1);
  },
  function() {
    $(this).find('.childItemToShowOnParentHover').fadeTo(500,0);
  }
);