Google introduced OpenSocial, a set of common APIs that make it easy to create and host social applications on the web. This will allow you to write an application once that will run anywhere that OpenSocial API’s are supported.
This will make it easier for you to focus on making your web apps better and it makes it better for users because they get more apps in more places. Members of the OpenSocial community include: MySpace, Engage.com, Friendster, hi5, Hyves, imeem, LinkedIn, Ning, Oracle, orkut, Plaxo, Salesforce.com, Six Apart, Tianji, Viadeo, and XING.
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.
OpenSocial - Google Code