The Emacs X Widget branch allows embedding GTK widgets inside Emacs buffers. Among the envisioned use-cases by the involved developers for this is embedding MPlayer's GUI into Emacs and one of the popular pursuits appears to be for embedding a WebKit-based GTK web-browser inside Emacs. But surely in the not too distant future we're likely to see other innovative uses for Emacs with now having the GTK widgets support.
An example from their Wiki of GTK widgets inside Emacs.
Details regarding the X/GTK widgets support inside Emacs can be found via this Wiki page.
This work has been ongoing for several years and then last month a discussion started about finally getting the code merged, which has now happened.
Yes, we are talking about the GNU Emacs text editor, the editor that prides itself on its extensibility and customizable options from GDB integration to file manager support to a email and RSS reader, built-in tetris game, and many other features built up over the past four decades.