Asterisk : From caller ID to long distance, anything your telephone system can do, Asterisk can do better and cheaper.
Asterisk was designed to be able to do everything a traditional telephone system can do, and much, much more. Asterisk is, at it's heart, a PBX system. However, it includes a whole host of telephony features such as voicemail and call conferencing. 

Speak-Freely is an Internet voice application that allows you to talk (actually send voice, not typed characters) over the internet.

SIP Communicator is an audio/video Internet phone and instant messenger that support some of the most popular instant messaging and telephony protocols such as SIP, Jabber, AIM/ICQ, MSN and soon others like Yahoo and IRC.

OpenWengo is a community of enthusiasts and developers, creating free software products related to communication over IP. The flagship product of the OpenWengo project is a softphone which allows you to make free PC to PC video and voice calls, and to integrate all your IM contacts in one place.