Depends on what YOU mean by monitor.
If you want to monitor which devices are connected, how long, what type, if they're reporting problems, look for software/hardare that implement SNMP (
http://en.wikipedia.org/wiki/Simple_Network_Management_Protocol ).
OTOH, if you want to monitor content, URLs, what ppl are browsing, what’s on their desktop, sniff traffic, etc., those are completely different type of monitoring.
So you need to be more specific.