I've heard good things about nagios,
and also Cactus.
At my work we have our own custom written monitoring software (monitoring thousands of devices). (Using SNMP and WMI as well as some custom script for pulling and parsing stuff from non-standard displays)...
depending on the brand of server or device you might want to try something like IBM director or HPs lights out manager, this works with non-IBM/HP devices too. but makes it a lot easier to monitor. (so rather than having to go an find (either by MIB inspection or SNMP walk) the mib that you're trying to monitor it loads mib files and you just select from a tree the stuff that you want to monitor... that might be CPU temp or chassis fan speeds). -which you can get using SNMP from ilo/RSA/imm/DRAC connections (assuming you have these plugged in and setup).