Firefox/Chrome: Sure it’s nice and all that we get Gmail for free, but those ads to the right of open messages aren’t really all that helpful. Free browser add-on Rapportive replaces Gmail ads with contact info about the sender.
Beyond installing it, there’s nothing to using Rapportive. You have to give it access to a minimum of information to get started (looks like your name and email address, via your Google account). Then whenever you open an email, it cross-references the email of the sender with various social networking sites, then fills the ad space with whatever information it finds.
As ReadWriteWeb points out, some users may be wary to trust a third-party add-on with their inbox, but your Gmail credentials don’t end up with Rapportive, and they don’t have any access to your password. My take: If you can get over the tin-foil hat concerns, it’s an extremely cool extension worth trying out.