Behaviour.js was my library for doing unobtrusive javascript using a combination of a CSS Selector library and some simple javascript. This functionality has now been adopted and improved upon by all the major javascript libraries.
I personally use Jquery, which through the .live() method, let's you do all of the functionality of Behaviour.js, but including automatically applying rules when new elements are added to the dom.
To paraphrase Zombieland:
“Jquery is like behaviour that has been sent back from the future by a secret fucking government lab”