Sipcat is a software IP telephony system that helps businesses increase productivity and reduce telecommunication costs. It allows you to start running your own voice over IP PBX in a matter of minutes. It contains a complete Linux distribution, including Asterisk and all required 3rd party software.
Skype uses P2P (peer-to-peer) technology to provide voice- and video-based communication with other Internet users. The technology is extremely advanced, but easy to use. It features excellent sound quality, end-to-end encryption, and automatic negotiation of firewalls or routers. Among major features are SkypeOut and SkypeIn, adding the possibility to make low-cost calls to land line phones and having a fixed number to be able to receive calls from land line phones.
Sofia-SIP is a SIP user agent library, compliant with the IETF RFC3261 specification. It can be used as a building block for SIP client software for uses such as VoIP, IM, and many other real-time and person-to-person communication services. The primary target platform is GNU/Linux. Sofia-SIP is based on a SIP stack developed at the Nokia Research Center.
SpeexComm is a Speex-based, multi-platform voice communication engine meant mainly for gaming. Written in C99, the engine itself depends only on libc, but ALSA and Speex libraries are recommended for audio input/output and voice encoding/decoding. The engine is accompanied by some reference UIs.
SurrealServices is a full replacement IRC Services package providing NickServ, ChanServ, BotServ, MemoServ, and OperServ, as well as LogServ and ConnectServ. Features are loosely based on ConferenceRoom/Auspice and Anope, as well as NeoStats. Some security features are also available such as a TOR (http://tor.eff.org/) detector/banner and a country detector (which checks IP allocation and optionally bans). It is multithreaded to eliminate lag due to the asynchronous nature of MySQL and DNS lookups.
SynchroEdit is a browser-based document editor for use by multiple users simultaneously. It is a form of same-time, different-place groupware. It allows multiple users to edit a single web-based document at the same time, and it continuously synchronizes all changes so that users always have the same version.
TChat is a chat applet that makes it possible for your site's users and visitors to communicate through the Web. The applet is completely customizable and can be confiured with your company logo and sponsors’ banners. The server supports unlimited connections, a multithreaded architecture, an access log system, a lightweight communication protocol, platform independence, and an administration console. The client features public chat with multi-font support, multicolor text and backgrounds, private chat with audio effects, support for running embedded in an HTML page or in a detached window, a shared blackboard function, and more.