There are several tools with which you can monitor your network.
First, what has Network Documentation to do with Network Monitoring?
More then you might think! Because you need to now the ins- and outs of your network you look at your doumentation to find out what critical devices you got.
Depending on that information you can start building a monitoring system.
Here are several tools that you will find very usefull:
Use the software embedded in your managed switch from HP,Cisco, 3Com and Nortel, that will provide you with information to troubleshoot a slow network.
Linux Based Monitoring Systems
Windows Based Monitoring Systems