Wikipedia
Tuitalk
Tuitalk is a free Voip client made by Argela. It allows users to call landlines and mobile phones via Wi-Fi. And, you are not asked for a credit card number. The application was released in 2008 for Windows and Mac OS X.
While the call was being made, the program shows a video ad before the call starts ringing. You can make only 3 calls a day, everyday for free.