Just a copy from Opensocial homepage:
The web is better when it’s social
The web is more interesting when you can build apps that easily interact with your friends and colleagues. But with the trend towards more social applications also comes a growing list of site-specific APIs that developers must learn.
Many sites, one API
Common APIs mean you have less to learn to build for multiple websites. OpenSocial is currently being developed by Google in conjunction with members of the web community. The ultimate goal is for any social website to be able to implement the APIs and host 3rd party social applications. There are many websites implementing OpenSocial, including Engage.com, Friendster, hi5, Hyves, imeem, LinkedIn, MySpace, Ning, Oracle, orkut, Plaxo, Salesforce.com, Six Apart, Tianji, Viadeo, and XING.
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.