I had the same question when Skype was discontinued, I did quite a lot of research on the alternatives (most of which have already been mentioned in this thread) but I ended up going for mytello. It works well, calls to UK and Australia are about the same price as Skype (the US even cheaper) and I've found the audio quality with mytello much better than Skype. It works in any country so I'm happy with the choice I made.