Windows Messenger 5.0 is the first client designed specifically for the enterprise environment and is built to take advantage of the enterprise features of the Live Communications Server.
This includes Active Directory integration, user search, roaming contacts, Microsoft Office 2003 integration and many other features.
Windows Messenger versions below 5.0 will not work with Live Communications Server. In addition to Windows Messenger 5.0, many partners have developed web and mobile clients taking advantage of Live Communications Server features, like IMlogic IM Manager & IM Auditor. A complete list of partner clients can be found here.