ThisNumber is an international directory of phone books and U.S. phone number lookup service.
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.
Truecaller - Find a person by a name or phone number worldwide for free using Truecaller.
CallerSmart - CallerSmart is an application that is basically used for the purpose of looking up mystery phone numbers for free.
Apple Contacts App - Access and edit your contacts from personal, business, and other accounts with the Contacts app.
WhosCall - WhosCall is a reliable caller ID app whose popularity has led to its adoption by many users including international media. You can now manage the numbers calling you regardless of whether they are part of your contact list or not... read more.
FullContact Card Reader - Automagically scan biz cards into LinkedIn/Gmail contacts