Online Multiplayer games are getting more and more popular, and communication is important when playing that kind of games in teams. Mumble is a multiplatform application, similar to Ventrilo, developed following the client-server structure that allows you to chat with all the players at the same time.
To establish communication you have to connect to one of the program´s servers. You can use a public one or create your own by using the application Murmur, included with Mumble.
Take note that if many users are talking at the same time the bandwidth consumption will be high enough (10-40 Kb per user)