In computing, a softphone is a software program for making telephone calls over the Internet using a general purpose computer, rather than using dedicated hardware. Often a softphone is designed to behave like a traditional telephone, sometimes appearing as an image of a phone, with a display panel and buttons with which the user can interact. A softphone is usually used with a headset connected to the sound card of the PC, or with a USB phone.
Note: Webcaster Kit
It's important to differentiate softphones from services based on softphones. Skype, Google Talk, and Vonage are Internet telephony service providers having their own softphones that you install on your computer.
Wiki http://en.wikipedia.org/wiki/Softphone
19 open source freewares for SoftPhone
X-Lite (free) http://www.counterpath.com/x-lite.html
VOIP Toolbox (including free web-based PC-to-Phone without software installation) http://mashable.com/2007/08/19/voip-toolbox/