SIP client for Mac OS X

On of the easiest SIP Phone I have stumble upon is SIP Communicator.  It seems that SIP Communicator remains the most active and solid open source (GPL 2) SIP client for Mac OS X. Builds are available in both Windows and Linux variants.   SIP Communicator is an audio/video Internet phone and instant messenger that supports some of the most popular instant messaging and telephony protocols such as SIP, Jabber, AIM/ICQ, MSN, Yahoo! Messenger, Bonjour, IRC, RSS and soon others like IAX.

SIP Communicator is a communication tool based on the SIP protocol, developed by the community and released under the GNU General Public License. It is more than just VOIP software,  it is a full-fledged communications tool that lets you enjoyfree calls, video conferencing, SMS, chat over many existing protocols (MSN, AIM/ICQ, Yahoo!, Jabber)...  Everything you need to stay in touchwith your friends and family!

SIP Communicator is available for download at:
http://www.sip-communicator.org/index.php/Main/Download

Setup a SIP line is easy.  See following setup typical Pfingo User.  Pfingo from Starhub provides FREE incoming calls and 1 cents per minute for outgoing calls in Singapore.