XMeeting is a SIP and H.323 compliant and Open Source
for Mac OS X (10.4 and later). The current version of XMeeting is 0.3.4. Historically, XMeeting
has evolved from ohphoneX. However, XMeeting has been rewritten from scratch and uses a totally
different software architecture than ohphoneX.
Although not yet being a 1.xx release, XMeeting is quite stable, has good interoperability with
other clients and provides many unique features such as the sophisticated Address Book
integration. Much effort has been put into the Mac OS X native GUI. We think the result is worth it!
As far as just client software for MACs, I've been using XMeeting.
http://xmeeting.sourceforge.net/ It has a few bugs here and there and
dose not look like its actively being developed, but overall its worked
well for me. The price is also right, FREE. (I'm on digest, so sorry
if this software has already gotten brought up.)
I've only used it for H.323 but it says it supports SIP. I've
personally connected successfully to everything I've tried. Polycom and
Lifesize endpoints, and Tandberg/Codian MCUs.
Our Video Conferencing person who deals with room systems, MCUs,
scheduling, etc, was very happy with the quality of the video I send
when connected with it. My machine I've been running it on is a 2 Ghz
Core Duo white MacBook with 2 GB of ram and 10.5.