Microsoft MOM, Compaq Insight Manager, HP Openview, IBM Tivoli....
Depends on what size of network you've got, and what kind of platforms/hardware you're running.
I personally like Insight Manager a lot. It's ridiculously powerful, but you'll have to find MIBs for your non HP Hardware (shouldn't be too hard to source out), or set up standard SNMP traps on it.
they're almost all linux boxes being monitored. an 'open' solution is really all I can use (budget). I tried out nagios and jffnms this weekend (shied away from opennms because, despite working at a java company, java apps give me the heeby-jeebies). nagios seemed really robust & mature, but I wasn't very impressed with it's snmp capabilities. jffnms has really blown me away, but it feels considerably less mature, and a lot of the features feel still 'under developement', but the sheer amount of info it's gathered from my network is impressive (as well, it's support for snmp/traps/remote syslog/etc. is really super). It's alarm features are great, if only I can figure out how to actually get it to send me emails.