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If you're a player who just wants to connect to other voice chats as needed, your best bet is to download Ventrilo and Teamspeak. These two options are the most popular among players hosting game voice chat servers, and having both installed will allow you to quickly connect to servers as needed.
If you're looking to buy or host a game server for your guild, either Ventrilo or Teamspeak will work, as well as Mumble. Teamspeak is generally not recommended for games that also support Mac players, simply because it will require players to download Teamspeex, which is not always compatible with Teamspeak servers.
Ventrilo is excellent as a background client, but for players who like to see who's talking, Mumble offers a visual overlay option. Ventrilo also offers several codec options, while Mumble uses Speex exclusively.
Skype is generally only recommended as an option for small groups of players, as it can only host 10 people within a single conference call.