http://mapawatt.com/misc/textarea.js?v=7.59

1(function ($) {
2
3Drupal.behaviors.textarea = {
4  attach: function (context, settings) {
5    $('.form-textarea-wrapper.resizable', context).once('textarea', function () {
6      var staticOffset = null;
7      var textarea = $(this).addClass('resizable-textarea').find('textarea');
8      var grippie = $('<div class="grippie"></div>').mousedown(startDrag);
9
10      grippie.insertAfter(textarea);
11
12      function startDrag(e) {
13        staticOffset = textarea.height() - e.pageY;
14        textarea.css('opacity', 0.25);
15        $(document).mousemove(performDrag).mouseup(endDrag);
16        return false;
17      }
18
19      function performDrag(e) {
20        textarea.height(Math.max(32, staticOffset + e.pageY) + 'px');
21        return false;
22      }
23
24      function endDrag(e) {
25        $(document).unbind('mousemove', performDrag).unbind('mouseup', endDrag);
26        textarea.css('opacity', 1);
27      }
28    });
29  }
30};
31
32})(jQuery);