Hunter - Find all the email addresses related to a domain
NeverBounce - Real-time email verification and cleaning to ensure emails never bounce.
AnyMailFinder.com - Find anyone's email address
Email List Verify - The Fastest Way to Improve Email List Deliverability and ROI
EmailMatcher - Find anyone's email address in a matter of seconds, for free
ZeroBounce - Removes invalid emails from your list to prevent email bounces from ruining your deliverability.