Teamspeak is definitely excellent. I'm a gamer, and my brothers and I use it to communicate. Personally, I think you could market it to several different areas, such as businesses, and more. Although it was conceived for gamers, it's more or less transcended to more of Voip software.
Besides Teamspeak, there's also Ventrilo. I've also heard of a third one coming, but Teamspeak and Ventrilo have the majority of the market.
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