Hiver - Manage Customer Support, Sales and Projects right from your Gmail inbox. Collaborate with Shared Labels, Shared Contacts, email notes, email reminders and email templates
Hunter - Find all the email addresses related to a domain
Clean Email - Clean Email is an online service that empowers you to take control of your mailbox.
AnyMailFinder.com - Find anyone's email address
MailClark - The Slack bot for external communications
EmailMatcher - Find anyone's email address in a matter of seconds, for free