The best replacement for office communicator is live messenger.

The best alternatives to office communicator are rated as following:
★★★★★  live messenger
★★★★  msn messenger
★★★★  lync
★★★★  live meeting
★★★  sametime
★  windows live messenger
★  lync 2010

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Speaking about office communicator, one probably shouldn't forget about pidgin.

Other search terms related to office communicator and their ratings are:
★★★★★ pidgin
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Google Trends

Scale is based on the average worldwide traffic of office communicator in all years. Click on graph for more.
office communicator
live messenger
msn messenger