Google’s page about it is at http://code.google.com/apis/opensocial/ and Marc Andreesen has a good writeup of the release on his blog at http://blog.pmarca.com/2007/11/report-from-the.html.
After I watched the video, though, I realized that Open Social doesn’t do what I was thinking about – which was to allow me to easily build a new web site that would reach into the social networks on the many sites supporting Open Social (like Ning, MySpace, LinkedIn, Orkut, etc.) to achieve functionality that leverages the social connections people have established in those spaces.
Instead, what Open Social is about is being able to write a single web application that can be embedded into each of those social networks’ own sites without having to be rewritten. That’s cool too, but not as cool as what I had hoped for.