You may love it, you may hate it, but if you're in a guild chances are you've used it -- voice chat.
One of the most popular programs for voice chat is Ventrillo, and finally a version that works with Macs has been released. Revel in the new-found knowledge that Konan, the uber level 60 tank in your guild sounds like a 13 year old who's voice is cracking, because that's exactly what he is!
I'm not crazy about voice chat because the chat tends toward the inane and reminds me of a phone call that never ends, but it will increase your effectiveness in team situations, such as taking on Molten Core or fighting in the battlegrounds. I'm willing to use it for that, but I keep it turned off otherwise.