http://en.wikipedia.org/wiki/Comparison_of_VoIP_software
http://en.wikipedia.org/wiki/List_of_SIP_software
Client on PC
http://ekiga.org/
Client on Android:
SipDroid
IMSDroid
Fring
Server:
http://www.freeswitch.org/
http://www.asterisk.org/ ==>PBX with call conference
http://www.elastix.org/
MySIPSwitch: http://mysipswitch.codeplex.com/
http://www.opensips.org/