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.
Search for more here:
http://freshmeat.net/
Note that M$ 'Office Communicator uses a proprietary instant messaging protocol. It may not be usable with non-Microsoft instant messaging clients'
source:
http://en.wikipedia.org/wiki/Microso...e_Communicator