If you put an Embedded Pen into a resizeable container, it will fluidly resize to fill both the vertical and horizontal space. Nothing overly incredible, but I link it up here because I think the whole idea of offering user-resizeable things on the web is interesting. There is a resize property just for this.
Clearly browsers thought offering it by default on <textarea>s was a good idea. What else makes sense? Maybe inputs? Maybe text containers so users can pick a line length that they like? Maybe this is another form of theming?