The best method would be to use CSS3. It supported by at least Webkit and Gecko.
According to the w3c spec:
div.my_class {
resize:both;
overflow:auto; /* something other than visible */
}
Webkit and Firefox do not interpret the specs the same way. In Webkit the size is limited to the width and height set. There’s another question: How can I use CSS to make an element resizable to a dimension smaller than the initial one? concerning this.