NeverBounce - Real-time email verification and cleaning to ensure emails never bounce.
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.
Email List Verify - The Fastest Way to Improve Email List Deliverability and ROI
Free Address Book - In our contact management software you can store fast and easily name, phone numbers, birthdays, address, company name, city, mobile numbers, insurance and loan info, e-mail addresses.
ZeroBounce - Removes invalid emails from your list to prevent email bounces from ruining your deliverability.
Clearbit - Clearbit provides Business Intelligence APIs