Maybe old news for some but I've just tried out Google Social Network and it's impressive.
It allows to literally port a social network onto your website without the need to ask people to re-register themselves on your site.
One other cool feature is the fact that wordpress and phpbb already come prepared for google connect which means a lot less worries about integration and more free time to work out the gimmicks of the site.
This is a very approach to the same concept implemented by Microsoft (and others) over a decade ago when Passport was first introduced but with the twist that you have so much more control over the login registrations and are allowed to do a lot more.
At the moment I don't see myself using often this tool but it's certainly something to keep in memory for future projects.
Very good google, keep it up.