It's a simple formula: build a product that people love, have a basic free version, charge users money for advanced features. The product you see now will most likely stay free forever, but we are planning additional premium features based on what our users are asking for.

You can see our analysis of freemium business models on the Rapportive blog: http://blog.rapportive.com/is-freemium-right-for-you

Edit: looks like rahulvohra and I had a little race condition here ;-)