Mumble is a voice chat application with a handy overlay feature that is useable without tabbing out of World of Warcraft.
It is free for download here:
http://sourceforge.net/projects/mumble/Connect and chat with other ponies using the following server information:
IP: 69.175.99.202
Port: 3006
A custom host name is pending, but it will be Cloudsdale.MumbleFrag.com assuming that it's approved.
25 user max for now, but that is easily upgraded if the need should arise. The server will be up 24/7 for your use.