Mumble is a VOIP package (voice over Internet protocol) that lets you talk in real time to other people, like Skype. If you play networked games, you are probably familiar with Ventrilo or Teemspex which are similar packages.
We have a license for the use of Mumble with our Minecraft server. You can chat through Mumble with other players on our server. Mumble can be downloaded free, here , and is available for Macintosh or Windows machines (heck, even Linux). I have madea video to help you set up your Mumble client once it's installed on your machine.