So I just read about a "new" gmail add-on (it's been out since March) that enriches your contact data in the right side nav-bar where the ads are. Sort of like Xobni (http://xobni.com) Rapportive scrapes social networks based on the email address of the person (either the "from" on an email, or any email address you mouseover.) It's implemented as a browser add-on, and in the 20 minutes I've been using it so far, it seems pretty neat. It cannot yet aggregate a single human's multiple email addresses to understand that they may all be the same person, but it does support a bunch of social networks (you can see the full list here: http://rapportive.com/help )
It's definitely an early stage product, but it looks very promising as a way to enrich the gmail experience. I think it will be interesting to see what kinds of additional features they support later on, when they have a freemium model fully up and running.