Teamspeak

Teamspeak (commonly abbreviated to "TS" in the game) is the voice-chat software Teamspeak. It allows vocal communication between players during gameplay. A client / server application that runs in the background allows players to discuss strategies and tactics in realtime, even during the battle, without time consuming typing. As communication is a huge factor on the effectiveness of a team, Teamspeak is often considered a necessity for serious PvP.

Another popular voice-chat program in Guild Wars is Ventrilo.