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Linphone is an internet phone or Voice Over IP phone (VoIP). With Linphone you can communicate freely with people over the internet, with voice, video, and text instant messaging. Linphone makes use of the SIP protocol, an open standard for internet telephony. You can use Linphone with any SIP VoIP operator.

Ekiga (formely known as GnomeMeeting) is an open source VoIP and video conferencing application for GNOME. Ekiga uses both the H.323 and SIP protocols. It supports many audio and video codecs, and is interoperable with other SIP compliant software and also with Microsoft NetMeeting.

Nimbuzz offers limited availablility free calls and limited availability free text messaging for users via 3G and WiFi. Calls to other Nimbuzz users are free. PAY to call mobile or landline phones for as little as $0.02/minute in bulk with NimbuzzOut. Free SIP to SIP calls.

Zoiper IAX & SIP multilanguage and multiplatform (Windows, Linux and Mac OS X) softphone is a VoIP soft client, meant to work with any IP-based communications systems and infrastructure.

What does it do:

MicroSIP is a free portable SIP softphone for Windows based on PJSIP stack. It allowing to do high quality VoIP calls (P2P or on regular telephones) via open SIP protocol. From cloud of SIP providers you can choose best for you, register account and use it with MicroSIP. You'll get free p2p calls and cheap international calls.

SeaFour is the SeaFour.club community's chat engine, powered by Node.js and Socket.io.It's a socket webchat supporting aesthetic username customization and stylized messages using a markup language similar to 'LISP', and supports a multitude of alternative clients. It uses a free-as-in-freedom license that enables users to modify the software to best fit their needs.

Fring is a VoIP (voice over internet) app that allows you to make 2-way video calls, voice calls and live chat for free, from your phone or iPod.

Free calls using open technology on Android. A heavily enhanced derivative of SIPdroid, adding support for ZRTP, SRTP, ENUM, ICE/TURN

Free and open-source SIP/VoIP Client.

ShMessenger Mobile is a free instant messaging client for mobile phones, which supports YMSG (Yahoo!), MSN and GTalk protocols. In just a few minutes, you can chat with your friends on Yahoo!, MSN or GTalk from your mobile phone.