Google’s social API group is pretty interesting (got a who’swho of alpha geeks on it). The biggest problem with testing seems to be that the API uses Google’s index, which doesn’t get updated that often, so if you make a change to your page, you can’t test it immediately, you have to wait for Googlebot to catch up. I guess there’s just not much you can do about that.
Oh, the irony!
http://www.google.com/search?q=Google%E2%80%99s+social+API+group+is+pretty+interesting+%28got+a+who%E2%80%99swho+of+alpha+geeks+on+it%29.+The+biggest+problem&ie=utf-8&oe=utf-8&aq=t
(Actually Google updates the index almost in real-time for sites that have feeds.)
Plaxo have an Open Social Graph demo which works in real time.