For those of us who remember JavaScript as a toy, only used for superfluous "DHTML", it's amazing to see it used as a real language -- capable of running an x86 emulator. If nothing else, people will have to stop pointing at Gmail as an example of an impressive client-side web application.
Open a small floating pane with common accented and special characters.
The Advanced obfuscated JavaScript analysis he links to at SANS is just as impressive/scary.
Incredible: a significant subset of Firebug, also compatible with IE and Opera, in a bookmarklet.