ABTO Software's VoIP SIP softphone.
Can be used to call sip numbers and landline numbers (if sip provider has this option).

Features:
voice calls;
video calls;
text chat (based on SIP MESSAGE request);
works with all popual SIP servers, including Asterisk/FreeSwitch;
make direct calls between clients without SIP server;
allows to use UDP/TCP/TLS protocols for SIP transport;
ability to make secure calls using SRTP/ZRTP;
supports STUN/ICE;
IPv6 support.