Google's AJAX APIs let you implement rich, dynamic web sites entirely in JavaScript and HTML. You can add a map to your site, a dynamic search box, or download feeds with just a few lines of JavaScript.
If you want to play around with them, try using the Interactive Code Playground. Plus, the Google API Libraries for Google Web Toolkit has bindings for the Maps, Search, Visualization, and Language APIs.
Create visualizations and reporting applications that access structured data in a common format.