The company writes on its site:
"Rapportive shows you everything about your contacts right inside your inbox. You can immediately see what people look like, where they're based, and what they do. You can establish rapport by mentioning shared interests. You can grow your network by connecting on LinkedIn, Twitter, Facebook and more. And you can record thoughts for later by leaving notes. Imagine relationship management built into your email. For free."
If you're on busy listservs like I am, it really helps to see people's Linked In info, Twitter and FB sites, blogs and more. See my example to the left using the Rapportive add-on. Rapportive got acquired by LinkedIn in February, and I can see why. It's perfectly complements LinkedIn's own drive to be more social and more connective.
You can also edit your own info so that others who use the tool see what you want them to see. That's "brand control" in a nutshell!
Read more about Rapportive at The Verge.