SIP Communicator - the Java VoIP and Instant Messaging client.

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 and soon others like IRC, and IAX.

SIP Communicator is completely Open Source / Free Software, and is freely available under the terms of the GNU Lesser General Public License.

Get Nightly Alpha Builds Here:
http://download.java.net/communications/sip-communicator/nightly/macosx/