Skype

Features: 

•  Skype to Skype IMs, video and voice calls are free* over 3G or WiFi. 
•  Make low-cost calls and SMS to mobiles or landlines from your Android.
•  Send pictures, videos and files to any of your contacts.
•  Enjoy high-quality sound when you call anyone else on Skype.
•  Talk face to face or show what you’re seeing with front and rear-facing cameras.

Required Android O/S : 2.1+

Download Links:
https://play.google.com/store/apps/details?id=com.skype.raider&feature=apps_movers_shakers