Fring: Get two-way video calling with Fring and Skype users over 3G or Wi-Fi long with other communication methods.
Skype: The official application from Skype allows you to make free Skype-to-Skype calls over Wi-Fi and still access their low international rates to call people in other countries.
TextPlus: Free App-to-App messaging as well as regular text messaging. Adds lots of other features such as communities and group chats.
Truphone: You can make free VoIP calls with Truphone, Skype and Google Talk via this app as well as other calls, SMS messages and more, and you can receive calls in the background thanks to the new multi-tasking feature.