Otherwise, the most common voice chat programs in use are Ventrilo, Mumble, and Skype. Ventrilo and Mumble are very much like text chat programs that use voice instead. Skype is more of an internet telephone service.
I know players who use voice chat as guild chat, which can cause a problem with non-voice-chat-using guildies. Many don't have microphones, have to be quiet, are shy, or otherwise can't or don't want to speak instead of type. A schism can occur between those who do use voice and those who don't. And nobody likes schisms.
When not being used for chatter, players use voice chat to coordinate group activities -- such as dungeons or pre-made PvP. In these cases, not being able to at least hear everyone speaking can make a player a liability to the rest of the group.
Do you use voice chat when you play? If so, which one and what do you use it for? If not, why not?