OT - Detect selected text with JavaScript

Hello,

Does anyone know if it is possible for JavaScript to detect what text  
is currently selected by a user?  I would like to allow the user to  
select some text and then grab that text using JS.

I saw these guys do it : http://www.enlighter.com/ but I'm not sure  
if its possible to do this using JS only or if it needs a browser  
extension.

Thanks!
--Dave.