Ventrilo has a free server license version, but you will need to read the Ventrilo licensing agreement to see if you qualify to use Vent for free. Installation instructions can be found at Ventrilo Setup
Ventrilo is meant to be better than Teamspeak, but many people will need to buy a license to use it legally.
Note that if you set up your own Ventrilo or Teamspeak server, it can use a lot of bandwidth.
Here are some links to use freely available Ventrilo servers.