Hunter - Find all the email addresses related to a domain
fruux - Free service that takes care of your contacts, calendars and more so you don't have to.
Snov.io - More leads. More sales. More revenue.
FullContact - Put the power of full contact information into your web or mobile apps.
MailDump Verifier - Powerful Emails Verifying Tool that helps verify email address or validate email addresses
Google Contacts - Google Contacts is Google's contact management tool that is available in its free email service Gmail, as a standalone service, and as a part of Google's business-oriented suite of web apps Google Apps.