Google+: Messenger 

Hangouts on your phone
In life we connect with others in all sorts of places, at all different times. And the connections you make unexpectedly are often the ones you remember the most. We think Hangouts should keep pace with how you socialize in the real-world, so today we’re launching it on the one device that's always by your side: your mobile phone. To get started, simply find an active hangout in the Stream, and tap “Join”: