Nagios is a system monitoring application, that works on *nix and Windows (but not limited to).
In our setup, we monitor, Servers (BSD, Linux & Windows), network switches and printers.
It can monitor only using SNMP or use an agent installed.
It has a large community and a big database of plugins.
You can setup a map of how servers are connected through switches and printers connected to printserver, vpn tunnel connections etc.
It's a good nice Free piece of software which also exists in a commercial version.