Rapportive will make it possible for you to find anyone's email. However, you will have to do some guessing work. If you can't find the person's email by Googling (more than often you will be able to find it), then try to guess the person's email. If you guess it right, Rapportive will display the contact's picture and information, verifying that you have guessed the right email.
Lets just say you want to reach Bill Gates. You can: