Zello is a free application for mobile phone (and PC) that allows one to turn phones into push-to-talk walkie-talkies. The software also allows the creation of public channels for simultaneously communication between a maximum of 100 users.
As long as you are in range of a Wi-Fi hotspot you can chat for free to any of your contacts who have also installed the app no matter where in the world they are.
It’s available for Andorid, BlackBerry, iPhone and the humble PC.
• Get it here: Zello walkie-talkie app