Rapportive is a free plugin for Firefox, Safari, Mailplane (a standalone Gmail application for Mac), and Chrome to replace you gmail sidebar with rich contact profiles. 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.