OpenSocial

OpenSocial provides a common set of APIs for social applications across multiple websites. With standard JavaScript and HTML, developers can create apps that access a social network's friends and update feeds.

In order for developers to get started immediately, Orkut has opened a limited sandbox that you can use to start building apps using the OpenSocial APIs.

OpenSocial is built upon Google Gadget technology, so you can build a great, viral social app with little to no serving costs. With the Google Gadget Editor and a simple key/value API, you can build a complete social app with no server at all. Of course, you can also host your application on your own servers if you prefer. In all cases, Google's gadget caching technology can ease your bandwidth demands should your app suddenly become a worldwide success.

http://code.google.com/apis/opensocial/