SER is iptel.org's SIP Express Router. SER is a free SIP server that can act as SIP registrar, proxy or redirect server. SER features an application-server interface, presence support, SMS gateway, SIMPLE2Jabber gateway, RADIUS/syslog accounting and authorization, server status monitoring, FCP security, etc. Web-based user provisioning, serweb, available.
GNU Bayonne is telecommunications application server. The primary included applications are a interactive voice response (IVR) telephony server and a voicemail server.
The OpenH323 Project is open source implementation of the ITU-T H.323 teleconferencing protocol.
Asterisk@Home is designed to take a home PC and turn it into an Asterisk PBX. It is an ISO distribution based on s based on RHEL 3 Linux. It includes a GUI and a management panel as well as a number of useful addons like the Festival text to speech engine. Kerry Garrison has a nice writeup on Building your own PBX with Asterisk@Home.
Xorcom Rapid is a customized Debian Linux distribution that automatically installs and configures the Asterisk PBX.
SIPatH (short for SIPatHome) is an open source VoIP router that runs on Broadcom based Linksys WRTs running OpenWRT. The router runs SER.
Skype voice chat, video chat and instant messaging.