AJAX push

We're writing a webserver in our spare time and we are now at the point where we want to explore new web features. What is the best way to do full asynchronous communication with javascripts? Can we just periodically check in javascript if we've received something? Or is this only possible if you do a XMLHttpRequest beforehand? Are there other methods?

>>18
Didn't read through all of the PDF, but it's rather annoying how Web 2.0 is all about using shitty "technologies" like AJAX (lololol let's make one HTTP request for every character you input in the search bar) or Comet or even Flash instead of doing something that's actually a good idea because all the PHP kids will get their websights "hacked."
I moderately mad.