Viber works well if you also want VoIP to POTS. Wire is free. The problem with all these things is the general lack of interoperability, so if you're not going VOIP to POTS, then you need several apps installed.
It's a real shame about Skype. I've used it almost exclusively for dialout for many years. Now it wants to be my friend, root through my contact list and tell me what all my friends are doing. Bring back the Bakelite phone with cloth-covered cord.