TeamSpeak 3
Well, I don't use TeamSpeak that often but I use Ventrilo much less due to it's ugly GUI. Though the wait for this is finally over, it's still in beta form but now apparently has an SDK(?).
Download the Client or Server (Note: Marked as a Temporary Site):
http://www.teamspeak.com/?page=teamspeak3
EDIT:
Upon looking at it, the GUI and Voice quality of TeamSpeak 3 compared to TeamSpeak 2 has improved a lot and to me it sounds better then Ventrilo. You can also set your own Avatar in TeamSpeak 3, but I haven't been able to do so in Freelancers server. The max size listed for an Avatar in the documentation is 300x300 (pixels). Added you can also 'Poke' other clients with a certain reason which gets displayed to you along with a sound notification of being poked.
Screenshots!
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