Audio Chat
Here's a few Audio chat applications that might be handy for either multi-bridge games or Internet based games:
TeamSpeak: http://teamspeak.com
Windows, Mac OSX, Linux, Android
Ventrilo: http://ventrilo.com
Windows, Mac OSX, Linux (unofficial Android client - Mangler)
Mumble: http://mumble.sf.net
Windows, Mac OSX, Linux (unoffical Android client)
Comments
Discord: https://discordapp.com/
Windows, Mac OSX, Android, Apple IOS (Linux coming soon) apps available. Alternatively you can run a cut-down app in Chrome, Firefox or Opera.