After several weeks of speculation, Yahoo announced that it is joining the MySpace and Google Inc OpenSocial Foundation to ensure the neutrality and longevity of OpenSocial as an open, community-governed specification for building social applications across the web.
Yahoo’s in its release announced that “Yahoo! believes in supporting community-driven industry specifications and expects that OpenSocial will fuel innovation and make the web more relevant and more enjoyable to millions of users.
Many sites, one API
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.
Source : http://code.google.com/apis/opensocial/