Mig33 is a mashup I came across at the O'Reilly ETel conference, which mixes Web-based social networks with cell phone technology in a very interesting way. Mig33 lets users create profiles and interact with friends from their phones or their Web sites. You can also text chat with people through the Web interface or your mobile phone.
There's also a VoIP angle, allowing you to make low-cost international calls using Mig33's phone app, or set up the call to connect on both ends using Mig33's third-party connection service. There's still some cost involved, but in some cases you can get about the same rate you'd get from a good calling card or Skype's SkypeOut service.