I actually hate this term coined by software publishing mogul, Tim O'Reiley, but I like the concept. Cyberpanopticon provides a link to a useful compendium of popular Web 2.0 sites.
Secondly, one of the sites listed was for the Google Web Toolkit. With this toolkit, coding Web 2.0 AJAX sites is much easier through Java wrappers. I only just discovered this, but I plan to spend significant time making use of it.