Sun ONE Identity Server 6.0
scubacuda points to this article at The Register, about "what is believed to be the industry's first identity server based on Liberty Alliance Project specifications for federated network identity (date sheet here). Other reports of Sun's release: eWeek, Information Week, Computer World, & Y!"
Notice that Sun and the Liberty Aliance do NOT offer a centralized identity repository a la Passport. This is a distributed solution -- as a provider (internet store, etc.) you install an authentication server and decide yourself what other providers you can trust, so that if a user is logged in their site, he can be considered logged in yours as well (put simply).
The only requirement is that the server implements the Liberty Aliance protocol standards. I _think_ one can make an open source server that implements those standards as well.