This project develops a third-party plugin for the Pidgin multi-protocol instant messenger. It implements the extended version of SIP/SIMPLE used by various products: Microsoft Office Communications Server (OCS 2007/2007 R2 and newer) Microsoft Live Communications Server (LCS 2003/2005) Reuters Messaging With this plugin you should be able to replace your Microsoft Office Communicator client with Pidgin.
WWW: http://sipe.sourceforge.net/