TeamSpeak and Ventrilo are popular Voice apps.
Teamspeak if you run your own server is very customizable.
For example
here is the Teamspeak Server that my friends and I use.
The visual list there is produced by TSViewer which offers free services for embedding pannels like that into your web page of which updates in real time (well once every 1min).
If you run your own server then it is possible to have as many channels and users as you like.
You can setup registered channels that cannot be changed and you can setup user permissions, you can password the server for private use only also.
Best of all, the channels are not tied to game servers, it is totaly independent.
All this is done via a web interface for the server side.
We have been using this for many years now as have found it reliable, good audio quality, and very easy to use!
I have a dedicated server in a datacenter in the UK and a backup server in the US.
We get good latency times so as not to have toomuch of an impact in game when shouting out commands like BAIL!! when your about to crash a plane or such hehe.
You can host this at home also of cause, although I would recommend the person hosting the server has a reasonable internet connection specially as the users start stacking up, you can of cause reduse the audio quality by changing the channels default codec.
There are also some providers online that host free channels for you to use, along with dedicated paid for services.
Should you like any assistence in setting up a secure TeamSpeak server feel free to give me a shout, as there are a few holes in the server which means it can be hacked.
However with some tweaks to the servers settings you can prevent this.
EDIT:- AussieTimmeh you got PM ;)