Fixing some of Jesse's code. Example of using jquery extend functionality.

modalize.js

$.fn.modalize = function(){
  $(this).each(function(i,v){ // Iterate over each of the elements
    var el = $(v), popup = el.prev('.popup');
    el.bind('input', function(){ // When the user types within input...
      if(popup.is(':hidden')) el.prev('.popup').fadeIn(100); // Grab previous sibling with class of 'popup'
    });
    el.bind('focusout', function(){ // When focus leaves this element...
      $('.popup').fadeOut(300); // Fade all popups out
    });
  })
  return this; // Return elements so you can chain methods - example: $('input').modalize().show();
};

$(function(){
  $("input").modalize();
});