I've never used Selenium but I guess it's for simulating user interaction in all browsers.
That's like integration tests.
But how do you test your js libraries/frameworks (unit testing) on all the browsers in an automated way?
The best one imo is the one from YUI : http://developer.yahoo.com/yui/3/test/ But doing unit-testing in every browser is kind of hard... Most people just test with it during development and just use node.js to test later on in case they broke something.