Mumble is available as a free download and source code is available on GitHub.

The server software Murmur is licensed under a free and open-source BSD-style license.

Available on Windows, Linux, OS X, Android and iOS which allows for the user to choose what operating system works for them and still be able to use Mumble.

Since version 1.2.4, Mumble uses an audio codec named Opus that's specifically developed to provide low-latency voice communication over Internet. This means that there is no delay time, while audio still comes out clear.

Dedicated servers and low overhead allows Mumble to have the least delay between users speaking and others hearing their speech, which can be very important for fast paced games.

A push-to-talk key allows you to only transmit audio when you want it. Others won't hear you chewing or your mom entering the room unless you want to.

Mumble is completely free, even hosting your own server.

Mumble uses certificates for user identification. A certificate can be generated by the user himself or issued by a third-party organization with organization certificates having 2 classes. A self-generated certificate is not tied to anything, a Class 1 certificate is tied to an e-mail address and a Class 2 certificate requires proof of identity. Depending on the server a different level of certification might be required.

Mumble server side is easy to setup and works smooth even on Raspberry Pi, which is pretty short on resources.

An in-game overlay can be enabled to show users in the channel or users talking. Overlay can be disabled completely or on a per-game basis by going to Settings->Overlay. This allows for users to see their chat but not obstruct the game they are playing.

By default, only users in the same channel can hear each other. Channels can be nested to better organize yourself according to what you want to be talking about.

This encryption is mandatory and cannot be disabled. The so-called control channel, which transports your chat messages and other non-time critical information, is encrypted with TLS using 256 bit AES-SHA. The voice channel carrying speech and positional audio is encrypted with OCB-AES 128 bit.

Mumble includes great automatic volume adjustment functionality that can be used to prevent big volume changes while still allowing good communication.

Mumble supports positional audio for over 50 games, including versions of Call of Duty, Battlefield and Counter-Strike. This allows you to hear the audio from the direction of the player speaking, making gameplay more immersive and giving competitive players an edge. You can find a full list of supported games here.

Version 1.3 introduced skins, allowing you to customize your UI, either on your own (tutorial here) or by using one of the skins found here.