The several people that run Ventrilo know that they basically have the monopoly on the voice chat servers and client for gamers. I'm sure Ventrilo does plenty of sales for other users too, though. I could see a lot of the "average" users just using something that comes with Windows, or something, though. Anyways, the Ventrilo guys stopped giving out server licenses a while back and upped the restrictions for people that held licenses. They're doing this to ramp up prices for the licenses and to make it a competitive thing for resellers. They're probably making a fortune doing this. This is what I've been told by the guys that run darkstarllc.com, anyways - they're a Ventrilo reseller.
There's no real competitor out there when it comes to gamers. Ventrilo is just what they use. I find this sad because to me Ventrilo is far from being perfect. It's hardly even a great piece of software when it comes to the client. There's several things that I run into basically during regular usage of the client that I always wish it could do. Ventrilo isn't really in a position that requires them to make any changes, though.
I've been putting together just some very simple proof of concept software today. I wrote a simple echo server, and have been transmitting encoded data to clients. I'm just working on turning that decoded data into actual sound. haha.
Edit: I've also been using Java because of how easy it makes exchanging serializable Java Objects. Not to mention the cross-platform pwnage. The Netty framework is pretty 2legit2quit, too.