From a Mac perspective, Netfone is a near-perfect choice -- OSX-native, capable of recognizing any system audio I/O device (it had no problems with a Logitech USB headset, which was necessary for my particular desktop rev), throttled bandwidth requirements, and a remarkably low CPU footprint given that it's doing real-time encryption.
Here's the caveat: since the current version of NetFone is peer-to-peer (an issue they're correcting in the next version), you can't currently "conference" in any meaningful sense. Instead, you'll have to coordinate multiple connections.
If you have a four-