Viber is an Android app which takes advantage of the VoIP feature in Android devices. It allows you to make calls and send texts completely for free.
Here is the market description:
Viber - Be Free to Communicate. Call and text anyone, anywhere.
Viber is an Android* and iPhone application that lets you make free phone calls and send free text messages to other users that have Viber installed. When you use Viber, your phone calls to any other Viber user are free, and the sound quality is much better than a regular call. You can call any Viber user, anywhere in the world, for free and now also text them. All Viber features are 100% FREE and do not require any additional “in application” purchase.