You do not need a third party app to place WiFi calls. It is a feature that comes standard with most phones, including iPhones and Android devices. Carriers like Verizon and T-mobile also support this capability without outside apps. Just enable the feature in your phone’s settings to use it.
However, there are apps you can use to place WiFi calls, like Google Voice, Skype, Tango, and Fring. These generally offer advanced features like international WiFi calling or the ability to call from other devices like laptops.