Javascript is always a nice topic to discuss. Depending on the technical level, you could take it in different directions:

* "good" paradigms in Javascript (using concepts of functional programming). I always recommend Crockford's Javascript: The Good Parts - http://oreilly.com/catalog/9780596517748

* Demonstrate some interesting JS libraries such as mootools or Raphaël - http://raphaeljs.com/

* For less technical folks, you could demonstrate new JS graphical capabilities, such as the porting of Quake and the chrome experiments gallery - http://www.chromeexperiments.com/