WengoPhone

WengoPhone is a SIP phone which allows users to speak at no cost from one's computer to other users of SIP compliant VoIP software. It also allows users to call landlines, cellphones, send SMS messages and to make video calls. None of this functionality is tied to a particular SIP provider and can be used with any provider available on the market, unlike proprietary solutions such as Skype.
SpeakFreely

Speak Freely is a 100% free Internet telephone originally written in 1991 by John Walker, founder of Autodesk. After April of 1996, he discontinued development on the program. Since then, several other Internet "telephones" have cropped up all over the world. However, most of these programs cost money. Most of them have poor sound quality, and don't support Speak Freely's basic features such as encryption, the answering machine, or selectable compression.
Gspeakfreely

Gspeakfreely is a VoIP system with a flexible component system. It implements a set of audio processing components which can be connected to each other or mixed together. The most important components are net in/output, which implement VoIP functionality and the OSS-DSP in/output component.

Additionally there is a ISDN in/output component that allows making actual phone connections, and a file input component that can also play Internet radio streams. Also included is a fading plug-in, that can for example fade incoming calls into your music. New components can be developed for specific purposes, and combined with existing ones.

The net in/output components also have conference support. The net input component can mix incoming audio data from different hosts.
linphone

linphone is a SIP webphone with support for several different codecs, including speex.

Linphone is a web phone: it let you phone to your friends anywhere in the whole world, freely, simply by using the internet. The cost of the phone call is the cost that you spend connected to the internet.

linphone features include:

* Works with the Gnome Desktop under Linux, (maybe others Unixes as well, but this has never been tested). Nevertheless you can use linphone under KDE, of course!
* Since version 0.9.0, linphone can be compiled and used without gnome, in console mode, by using the program called "linphonec"
* Works as simply as a cellular phone. Two buttons, no more.
* Linphones includes a large variety of codecs (G711-ulaw, G711-alaw, LPC10-15, GSM, and SPEEX). Thanks to the Speex codec it is able to provide high quality talks even with slow internet connections, like 28k modems.
* Understands the SIP protocol. SIP is a standardised protocol from the IETF, that is the organisation that made most of the protocols used in the Internet. This guaranties compatibility with most SIP - compatible web phones.
* You just require a soundcard to use linphone.
* Other technical functionalities include DTMF (dial tones) support though RFC2833 and ENUM support (to use SIP numbers instead of SIP addresses).
* Linphone is free software, released under the General Public Licence.
* Linphone is documented: there is a complete user manual readable from the application that explains you all you need to know.
* Linphone includes a sip test server called "sipomatic" that automatically answers to calls by playing a pre-recorded message.