IP Telephony applications.
FreeSWITCH
A Cross-platform, Scalable, Stable Multi-Protocol Soft Switch http://www.freeswitch.org
Asterisk
A software-based, Open Source Converged PBX system that has revolutionized the traditional PBX industryhttp://www.asterisk.org
Callweaver
Fork of Asterisk PBX system , Best to support faxing http://www.callweaver.org
Kamailio
SIP proxy server, call router, and user agent registration server used in Voice over Internet Protocol and instant messaging applications. http://www.kamailio.org/w/
OpenSIPS
SIP proxy server, call router, and user agent registration server used in Voice over Internet Protocol and instant messaging applications. http://www.opensips.org
Sipexes
sipX ECS (Enterprise Communications Server) is SIP based voice over IP telephony server, an ideal IP based communications system (IP PBX) http://www.sipfoundry.org
Gnugk
H.323 gatekeeper based on the OpenH323 stack. A gatekeeper provides address translation, admissions control, call routing, authorization and accounting services http://www.gnugk.org
OpenSBC
Session Border Controller and B2BUA featuring Registrations, B2BUA, NAT traversal and ENUM http://www.opensipstack.org
Asterisk GUI interfaces.
FreePBX
A full-featured PBX web application supporting both asterisk and freeswitchhttp://freepbx.org
Elastix
An open source Unified Communications Server software that brings together IP PBX, email, IM, faxing and collaboration functionality http://www.elastix.org
TrixBox
Web based asterisk PBX managment software utilizing best open source tools built over asterisk http://www.trixbox.org
PBXinaflash
Similar to Trixbox and Elastix it is web based asterisk PBX managment software built over asterisk http://pbxinaflash.net/
Vicidial
(Astguiclient), a professional web based inbound / outbound call center solution built over asterisk http://astguiclient.sourceforge.net
Billing Systems.
A2billing
A2Billing combined with Asterisk is a physical Telecom Platform and Soft-Switch providing a wide range of telecoms services using both traditional telephone technology or VoIP. It contains a real time billing engine http://www.asterisk2billing.org
ASTPP
, A calling card voip solution built over asterisk and freeswitch http://www.astpp.org
AstBill
(AstBilling) , A Routing and Call Managment Billing solution built over asterisk and drupal http://astbill.com
Freeside
Freeside is the premier open-source billing, CRM, trouble ticketing and provisioning automation software for wired and wireless ISPs, VoIP, hosting, service and content providers and other online businesses. http://www.sisd.com/freeside
CDRTool
a rating and mediation engine for Call Detail Records generated by opensips and others http://www.ag-projects.com/CDRTool.html
ICTFAX
An email to fax gateway, supports G.711 faxing , PSTN faxing and T.38 origination and termination .ICTFAX is complete faxing solution http://www.ictfax.org
CitrusDB
A Multiuser Billign solution for internet service, subscriptions, consulting, and telecommunications , It Provides CRM, Ticketing, Invoicing, and Credit Card Batches http://www.citrusdb.org/