Ventrilo is a very popular Teamspeak-like voice application for Windows and Linux. They have stated that a Mac client was in the works but that was over a year ago. I checked on their progress today and found an update from May 30th.
I am very interested in this because I have some issues with Skype
This is promising news, but as with most software ports of this kind cough Teamspeak cough I will believe it when I see it.
There may be another option as well, but that will require some testing.