gistfile1.js

$(function(){
	var _new_li = new String();
	var _element = new Object();
	
	$( "#draggable > li" ).draggable({ revert: true });
	$( "#droppable").droppable({
		accept: "#draggable > li",
		drop: function(event, ui){			
			console.log(ui);
			_new_li = $('<li>'+ui.draggable.context.innerHTML+'<input  name="campaign[employee_ids][]" type="hidden" value="'+ui.draggable.context.id+'" /> <span><img src="/assets/cancel.jpg" alt="Remover" title="Remover"></span></li>')
			.attr('id', ui.draggable.context.id);
			
			
			$('#droppable').append(_new_li);
			$(ui.draggable).remove();
		}
	});
	
	$("#droppable > li > span").live("click", function(event){
		var _element = $(this).parent();
		
		
		_element.children().remove();
		_new_li = $("<li>"+_element.text()+"</li>").attr('id', _element.attr('id')).attr('class', 'draggable');
		
		_new_li.draggable({ revert: true });

		$("#draggable").append(_new_li);
		_element.remove();
	});

});

But you overwrite the values of _new_li on lines 10 and 24, and of _element on line 20. The String and Object are thrown away there. And I don't see any need for the variables to be actually instatiated in other places.