Ventrilo
Ventrilo is a VOIP program for your computers that allows you to talk to other people over the internet in real time. This program is commonly used by gamers who play games such as World of Warcraft because it allows them to organize themselves easier.
UGT-Servers offers ventrilo hosting which means they host the ventrilo servers you talk on. The service is very affordable, starting at $2.69 a month for unlimited talk.