There are a number of email plugins that look to give you contextual information about the person you're communicating with. The first one I tried (and arguably the best I've seen) is Xobni, an Outlook plugin. There's now a similar plugin available for Gmail users called Rapportive. Rapportive replaces the ads you normally see in the right-hand sidebar with a profile of the person you're emailing with that is automatically generated by searching online services for your correspondent's email address. Rapportive is only available to users that are using either Firefox or Chrome as their browser, since Firefox and Chrome have a plugin architecture. Read more: Downloadsquad
Download: Rapportive
Download: Rapportive