Google Base is a great example of an XML application. It uses RSS to create new types of content. Google Base comes with a set of pre-defined item types. You can create your own.
Let us look at an example. I buy a lot of used books from Amazon. However, sometimes I pay more money for shipping, than the cost of the book itself. I do this because the combined cost of the used book and shipping is still lower than the price of a new book from a nearby shop.
Now look at the future with Google Base. Let us say we create a type called Used Books in Google Base. Every one lists their books using this type. All the books go into a giant database. Now I can go to Google and look for the books I want. Using location information, Google can show me the nearest used bookstore where my book is available. No intermediaries, no shipping costs. I can give business to my local book stores, I can find something I want fairly fast and pay even less than what I pay today.
Google Base enables a new brand of applications. These are made possible by a unique combination of a search engine, a mapping tool, an online database and some cool XML technology.