I use Redphone almost everyday. They now have global coverage except for most of the Middle East and some parts of Eastern Europe. The Egyptian server apparently doesn't relay so well either last I heard on the mailing list.

Works as advertised in China which is great success for businesses worried about their blackberries or iphones being exploited by industrial espionage.

Another bonus is moxie is considering moving everything over to voip so you won't even need a cell contract (since google voice numbers don't work worldwide). Move around the city like a ninja using wifi