ExInsight is a free monitoring and statistics tool that provides a real-time view of the internals of Exchange Server transactions and allows you to evaluate individual user’s usage of Exchange Server.
This tool is especially useful for Exchange administrators and developers. By using this tool, you can gather real-time data about the users in an Exchange environment and learn more about the inner workings of MAPI, OWA, POP3, IMAP4, NNTP and other protocols.
ExInsight helps you to understand how individual users affect the performance of an Exchange Server, analyze application usage of Microsoft Exchange, and guide troubleshooting of applications that depend on Microsoft Exchange.
I have found this tool to be invaluable for troubleshooting Exchange issues.
Download ExInsight from here