Rapportive replaces Gmail ads with useful social information
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.
The information Rapportive provides is interesting, and it searches a surprising number of online services. But I can't help but think that they're putting themselves on shaky ground by choosing to replace the ads in Gmail; I doubt Google could easily block this behavior technologically, but if Rapportive gets too popular I don't doubt that Google's lawyers might be able to block it, permanently.
In the mean time, if you're looking for a clever way to get more information about the people you correspond with the side-effect of blocking ads in your Gmail, have a look at Rapportive.