Rapportive is a good web service. It’s a browser extension for Gmail that puts information about correspondents in a sidebar. Here’s an example screenshot. It shows Tim’s face, his location, his jobs, and details from social media like Twitter, Facebook, etc.
The UI is quite nice, the way it sits next to my email without calling attention to itself. I regularly find helpful context on random people in my mailbox. The data mining is pretty good, I suspect they’re leaning heavily on LinkedIn for location, titles, etc. Gmail is the current application but the profiles they’re building on people could have enormous value in a variety of contexts.
Apparently I’m late to the party; they got all their press in 2010 and were bought by LinkedIn last year. Not sure why I hadn’t heard of it before. It’s a bit uncomfortable how deeply it links into Gmail, but it’s useful enough I’m giving it a try.