We have Mumble installed on the server. This is a simple but high quality voice chat program like Ventrilo, TeamSpeak and Skype. It is very simple to use and is basic to install. Anyone is allowed to use mumble, but please note that all Rules from the Minecraft server apply on Mumble as well! This includes swearing and respecting moderators.
You can download Mumble here. When setting it up, please make sure you use 'Push To Talk' instead of automatic voice detection, otherwise your microphone can spam other users. Server details are outlined below:
Port: 64738 (default)
Password: no password
If you have any problems getting it set up, please ask a Moderator for advice.