IP Telephony applications.
FreeSWITCH http://www.freeswitch.org/
Asterisk http://www.asterisk.org/
OpenPBX http://www.openpbx.org/
SSR http://iptel.org/ser/
OpenSER http://www.openser.org/
SipX http://www.sipfoundry.org
ReSIProcate http://www.resiprocate.org
Gnugk http://www.gnugk.org/
Vicidial (Astguiclient) http://astguiclient.sourceforge.net/
OpenSBC http://http://www.opensipstack.org/
Asterisk Gui interfaces.
Voiceone http://www.voiceone.it/
FreePBX http://freepbx.org/
Billing Systems.
Asterisk2billing (a2billing) http://www.asterisk2billing.org/
ASTPP http://www.astpp.org/
AstBill (AstBilling) http://astbill.com/
Freeside http://www.sisd.com/freeside/
CDRTool http://www.ag-projects.com/CDRTool.html
Ispbs http://ispbs.hostplus.net/
Trabas http://www.trabas.com/opensource/
Agilevoice http://www.agilevoice.com/