Use Element's prototype to extend its functionality:

Element.prototype.myNewFunction = function() { 

      // your code...

};

Now you can call this method on any element object.

Edit: I've just had a quick check around, and it appears that this will not work for IE7 and below, and IE8 might be iffy.

Edit 2: Also as Eli points out it's probably not best practice to extend objects you don't own. Given this and shaky IE support you might want to consider a simple procedural function:

function myFunction(element) {

  // your code...



  // which may or may not return an object or value

  return blah;

}