TeamSpeak 4 Free claims to have free TeamSpeak servers available for guilds etc to use. I haven't look into it. I play WoW, but given what I hear on channels, I have no wish to listen to people on TeamSpeak *shudder*. You might find it useful though.
Ventrilo is meant to be a nicer system to use than TeamSpeak. See here for free Ventrilo Server information
The sad fact of the matter
The sad fact of the matter is that any "Free" Teamspeak hosting solution eventually gets shut down. It might take a month or 2 but the people over at Teamspeak blacklist Servers all the time. They do this because they need to feed their families. In fact if you look through this entire thread you will see that most of these IP, Website, and other Items people are giving you either have shut down cost lots of money or have been blacklisted
The people over at TserverHQ(dot)com used to host around 1000 slots for Free. Teamspeak shut them down for about a week and then they came back. To be able to legally still give out free Teamspeaks they are doing 2 things. They are giving away a free Teamspeak to anyone that registers on their website each month and they are selling servers at cost. I got my 50 slot server for $0.18 a slot.
Check them out
(a little tip they have been hosting for awhile but they have a new site and I don't think they have that many users yet so you might get a free server right off the bat).
Sadly this is the only REAL way to get a reliable TS that will never be shut down by the owners of Teamspeak.
They are pretty friendly over at Tserverhq were when you have a question or submit a ticket so check them out
tserverhq(dot)com