Remember Microsoft Passport, er now Windows Live ID? Google has just introduced their entry in the space, which is now more of the Identity 2.0 space. For details see thier new Account Authentication Proxy for Web-Based Applications page. And check Garett Rogers post at ZDNet for a concise summary.
It was only earlier this week there were a couple of interesting posts on Web APIs and authentication worth reading: Julien Couvreur's piece on Web API authentication for mashups with follow-up by Dare Obasanjo. As Dare notes, "The devil is in the details when talking about authentication, authorization, and Web APIs." This is indeed one of the biggest obstacles in making mashups a more serious business.