Teamspeak
From #openttdcoop wiki
We run a Teamspeak Server for communication.
You find Clients for Windows, Linux and MacOS at http://www.goteamspeak.com/?page=downloads.
#openttdcoop.ts
Host: ts.openttdcoop.org (hosted by http://openttd.dihedral.de)
Port: 8767
Password: coop (connect as Anonymous - but please do connect with the same 'nick' as on irc or ingame)
Language: English