Hunter - Find all the email addresses related to a domain
PagerDuty - Cloud based monitoring service
AnyMailFinder.com - Find anyone's email address
OpsGenie - Alerting and On-Call Management for Dev&Ops Teams
EmailMatcher - Find anyone's email address in a matter of seconds, for free
Squadcast - Automate incident response, reduce downtime and enhance your tech teams’ delivery with a unified platform.