Next to Gartner Group (focussing on Unified Communications rather than 'just" Enterprise IM), Forrester Research recognises Microsoft as a leader in the field of Enterprise IM :
In the fall of 2007, Microsoft released Office Communication Server (OCS) as a major upgrade to its existing Live Communication Server offering. New to OCS are on-premise Web conferencing, videoconferencing, improved presence capabilities, and a host of partnerships to extend the value of the underlying platform. With excellent integration into Office 2007 and SharePoint and a strong unified communications strategy, the value of OCS extends well beyond basic chat.
Source: Microsoft Is A Leader In Enterprise Instant Messaging
Author: "Rob Koplowitz"
Date Published: Tue, 30 Oct 2007