Ever wanted to know what users doing to your Exchange server? If you're a serious admin you should want to know.
Most common is to use performance monitor to monitor all kinds of statistics and how much pressure there is on your Exchange server. You can for sure see how many users and how much load they generate on your server, this is all good but you cannot see what individual users are doing to your server which is really nice in case you need to troubleshoot for example performance issues.
Luckily there exists tools out there that monitoring MAPI clients in real time (well almost anyway) and the best thing is that they are free of charge.
Microsoft has one tool called "Microsoft Exchange Server User Monitor" or ExMon for short.
It has been around for a couple of years. It can monitor Exchange 2000 and Exchange 2003 and in the latest version even Exchange 2007.
Read more about the tool on Technet http://technet.microsoft.com/en-us/library/bb508855.aspx and the download link is http://www.microsoft.com/downloads/details.aspx?displaylang=en&FamilyID=9a49c22e-e0c7-4b7c-acef-729d48af7bc9
Unfortunate there seems to be something wrong with the download and you will not get the latest version that works with Exchange 2007, but I can only guess that this little glitch will be corrected shortly.
Recently I came across an almost identical tool "ExInsight for Microsoft Exchange".
You can read more about it and download it from http://www.bitrunes.com/
Both tools has there advanteges so there is no number one, they simply have different featue sets.