We have TeamSpeak installed on the server. This is a simple but high quality voice chat program like Ventrilo, Mumble and Skype. It is very simple to use and is basic to install. Anyone is allowed to use TeamSpeak, but please note that all Rules from the Minecraft server apply on TeamSpeak as well! This includes swearing and respecting moderators. You should also review the Teamspeak rules.
You can download TeamSpeak 3 here.
When setting it up, please make sure you use 'Push To Talk' instead of automatic voice detection if you have a lot of background noise, otherwise your microphone can spam other users.