If you've ever wanted to use a JavaScript library like script.aculo.us, Prototype, jQuery or Dojo, but you couldn't upload its files to a site or you found it difficult to manage all the different versions, Google hosts them for you.
"The AJAX Libraries API is a content distribution network and loading architecture for the most popular open source JavaScript libraries. By using the Google AJAX API Loader's google.load() method, your application has high speed, globally available access to a growing list of the most popular JavaScript open source libraries. (...) The AJAX Libraries API takes the pain out of developing mashups in JavaScript while using a collection of libraries. We take the pain out of hosting the libraries, correctly setting cache headers, staying up to date with the most recent bug fixes".
In addition to Google's APIs for search, maps, feeds and translations, you can now access AJAX libraries that add useful features like drag-and-drop, controls, animations or easier DOM manipulation. You can use Google as a fast proxy, but Google also caches the files related to iGoogle gadgets, the feeds that are served by Google Reader, and we should expect to see more content that can be accessed from Google faster and more reliably. In the future, Google could provide even more tools for a site: stats, monetization, search are useful, but what about using an unified Google API that lets you find facts, get spelling corrections, add ratings and forms, store data, add social features and code applications that run on Google's platform?