Cardhop - The Mac contacts app you'll want to use.
Hunter - Find all the email addresses related to a domain
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.
AnyMailFinder.com - Find anyone's email address
Contalist - The contact app for professionals and small businesses
Norbert - Norbert is an online tool that helps you to find anyone’s corporate email address.