Google’s OpenSocial documentation is here with an example. From Hosting OpenSocial Apps:
Your website can host third party OpenSocial apps integrated with your site’s social network. There are 2 steps to doing so — become a gadget container, and implement the OpenSocial Service Provider Interface (SPI).
This is as exciting as writing apps for the platform. This allows any one building their own social network to take advantage of the app community.