Nimbuzz, the free calls & universal chat solution from Rotterdam, has just launched an iPhone app. You can get it in the Apple iPhone store or download it here. Since they partnered with StudiVZ over 10 million people login per day (making it more popular than Skype???) and they add 10,000+ new users every day.
I downloaded the app on my iPhone and the integration with social networks (MSN, Yahoo, Facebook, Myspace, StudiVZ, Hyves) and Chat networks (MSN, Skype, Google Talk, ICQ) is seamless. Just enter your account data for any network and all your contacts from those networks will show up. Calling/chatting with them is as easy as clicking their names.