I am not sure, but i think its not open source software.
hmm, if OS is a requirement then it would have been best to state that in the initial post, might want to post ALL requirements you have, that way we can help you best and not suggest systems that are not suitable
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I have used LiveZilla for several months but It was little... too complicated for me. I would prefer to write own piece of software (we developed our own simple live chat software based on AJAX/XML) which perfectly fits our needs.