If you've got Firebug installed on your iGoogle page, you may have noticed how requests to fetch people, friends, or persistent data are formatted. These requests follow the RPC specification that's part of OpenSocial, and, while they've been part of the sandbox for a few months now, never really deserved a mention, until today.
To get started, download the client of your choice and try the included samples, which work on a variety of OpenSocial containers. (For reference, the iGoogle endpoints are http://sandbox.gmodules.com/api for REST calls, and http://sandbox.gmodules.com/api/rpc for RPC calls.) When you're ready to do your own iGoogle development, register your application to get the authentication keys you'll need to use the API.
If you have questions or comments, join us in the OpenSocial client libraries group. Have a RESTful (and RPCful) holiday season!