We use a few different systems depending on what you are trying to accomplish.
1. IPSwitch's
What's Up Gold. This lets us monitor our network/servers mostly via SNMP
2. A combination of
Automated Logic and
Essential Monitoring to keep track of the facilities side of things. (Gen, ATS, HVAC, UPS).
We tie the two together to give us one look at both facilities and network within our NOC.