Hunter - Find all the email addresses related to a domain
Clearbit - Clearbit provides Business Intelligence APIs
AnyMailFinder.com - Find anyone's email address
ZoomInfo - ZoomInfo is a B2B database providing detailed business information on people and companies.
EmailMatcher - Find anyone's email address in a matter of seconds, for free
InsideView - InsideView aggregates and curates all the company and contact data, news and social insights, and professional connections you need to do business better.