Rapportive is awesome because its simple and easy to understand. Everyone gets it without any confusion. Would be interesting to see how they make money, though. Thats a toughie, especially considering the target market is Gmail users.

Also, anyone know how they are getting Twitter, Facebook, LinkedIn and Skype info for email addresses? Are they using RapLeaf? If so, are they paying for every such lookup? That would be a huge cost to deal with.