HotSaNIC is a very nice tool which combines all sorts of very important system graphs into a simple and easy to understand webpage. It allows the admin to take a quick glance at the graphs to see what may or may not be working with the system. With the new APPS graph it is also possible to watch as more processes are started which can be helpful in tracking down why the server load is increasing. For instance if you see a huge load spike but you see that the number of exim processes has gone up significantly at the same time you can start to investigate.
I built this guide on a cPanel server but it will work fine on an ensim box as well. I don't recall the plesk mailserver off the top of my head but changing that small part will make it work great fine plesk.