Mumble
Mumble
I haven't used it myself—it seems to be a Ventrilo or Teamspeak alternative, functioning as a VoIP client.
Also I’m just going to share this, chat interaction is the best thing ever. I don’t know how many times it’s bothered me when someone in the chat starts eating and slurping loudly, making it hard for everyone to hear. If you’ve set the button to mouse button 5, it’ll be much better than pressing Z or X or something similar.
I utilize it and it performs excellently. I've never encountered any issues.