If you're an administrator on the network and don't mind spending a little time setting it up, a Linux Server running Nagios is great for monitoring all kinds of things on a network. The software is free to download from any number of websites

My company uses this software on our network of about 10,000 nodes.

The only disadvantage is the overhead on the setup. you pretty much have to statically assign every individual host or device that you want to monitor so you're probably only going to monitor critical resources.Nagios can monitor disk usage, traffic congestion, just about anything you can think of, plus you can download more scripts or write your own as your needs change.

I guess I should point this out, this software will not help you if you are "probing" or "auditing" someone else's network because you pretty much have to give it all it's information, it's not going to go out and set itself up.