Asterisk is a free and open source framework for building communications applications and is sponsored by Digium.

Asterisk turns an ordinary computer into a communications server. Asterisk powers IP PBX systems, VoIP gateways, conference servers and is used by small businesses, large businesses, call centers, carriers and governments worldwide

This video shows you the basic steps for installing AsteriskNOW. I am using this for testing IP PBX and SIP, but it can be used for much more.

You can download this from http://www.asterisk.org/