Skype

Experience free, unlimited, high-quality voice communication over the Internet.

This IP telephony software allows you to speak over the Internet (computer-computer). Moreover, a slow dial-up connection is enough for high-quality communication. It can also work via NAT firewalls. To ensure your privacy, all communication and messages are encrypted. Besides voice communication, Skype allows you to exchange instant messages, send text messages and also supports video calls, including video conferences.
Another important feature is the possibility to dial any phone number worldwide, but this feature is not free (though it is cheap).