Hunter - Find all the email addresses related to a domain
Crunchbase - Crunchbase is a platform for discovering industry trends, investments, and news about companies around the world.
AnyMailFinder.com - Find anyone's email address
CB Insights - CB Insights is a company database for the venture capital and corporate development community.
EmailMatcher - Find anyone's email address in a matter of seconds, for free
Owler - Owler is a crowdsourced data model allowing users to follow, track, and research companies.