TeamTalk

TeamTalk is just like the other millions of similar programs, except for its high quality voice conversation audio for several participants. The program uses up a lot of the computer's resources. But (probably because of this) you'll be able to talk with several friends while enjoying excellent sound quality.

You can also instant message the people connected to your local network, or other contacts whose IP addresses you've added

If your Internet slows down, the program allows you to lessen the sound quality so that your connection doesn't go out. Convert your Mac into a phone without spending a euro!