It depends on what you want from it. One of 'instant messaging (IM), voice over Internet protocol (VoIP) and videoconferencing.'
I would use ekiga for VoIP/Videoconferencing or there are other alternatives like Wengophone, etc.
For instant messaging you can use pidgin, kopette, etc.
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Note that M$ 'Office Communicator uses a proprietary instant messaging protocol. It may not be usable with non-Microsoft instant messaging clients'