ViaVoip
A portable peer to peer voice-chat/walkie-talkie.
ViaVoip is a simple Voice Over IP application that can be used when you need to talk, chat, or send files through the internet, but you can't or don't want to make use of any third party services.
Its peer to peer design allows the two end points to connect directly to each other, without any central server nor account registration.
It runs on Windows, Linux, Mac OS X and Android, and is portable, that is you don't need any setup, just get a copy and run it from any storage device.
It is free and open source.
Please read the last post for updates before downloading: -> https://sourceforge.net/p/viavoip/blog/.
You can check for version upgrades via the (i) button from time to time.