Rapportive is a browser plug-in for Firefox, Safari and Chrome that offers further context on the people emailing you by adding related links and information in a side panel in Gmail. The service adds contact links for Twitter, Facebook and LinkedIn, other links to sites like YouTube, Flickr and Quora and recent emails. Rapportive also offers Raplets, plug-ins that add more specific types of content, like your contact's Klout score, Lanyrd info, related Crunchbase information. If you're a developer, you can even create your own Raplet.
You'll quickly see the value in the service when you're in need of a bit more information on one of your contacts and don't have the time or bandwith to search for, say, their the professional history (via LinkedIn) or their social media prominence (via Klout). For more info, check out this video from Raplet:
Google has a similar tool called People Widget, which I'm not overly familiar with. What about you? What do you think of Rapportive and other related tools? Have they proven useful in your work?