fruux - Free service that takes care of your contacts, calendars and more so you don't have to.
Truecaller - Find a person by a name or phone number worldwide for free using Truecaller.
FullContact - Put the power of full contact information into your web or mobile apps.
CallApp - Free Caller ID & Call Blocker app that allows mobile users to block phone calls, identify calls, blacklist unwanted callers and much more.
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.
CallerSmart - CallerSmart is an application that is basically used for the purpose of looking up mystery phone numbers for free.