Of the "web 2.0" developments I think that the coolest is the explosion of "mashups" that resulted from companies like Google opening up their API (whatever that is) and letting any Joe Citizen develop a widget or service incorporating its service.  A great example is this mashup of data on North Carolina state parks and Google Maps.  If you click on any of the bubbles it will pop up a window with a weather forecast from Yahoo, a link to pictures tagged with the name of the park on Flickr , and a link to the park’s own website.  Simple, but effective.