Truecaller - Find a person by a name or phone number worldwide for free using Truecaller.
CallApp - Free Caller ID & Call Blocker app that allows mobile users to block phone calls, identify calls, blacklist unwanted callers and much more.
FullContact Card Reader - Automagically scan biz cards into LinkedIn/Gmail contacts
CallerSmart - CallerSmart is an application that is basically used for the purpose of looking up mystery phone numbers for free.
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.
Sync.ME - Search over 1 Billion phone numbers worldwide. Identify unknown phone numbers and block unwanted calls.