There is no other NetApp tool to monitor your filers besides operations manager.
There are other monitoring tools available tho like Microsoft System Center Operations Manager or the open-source solutions like nagios, cacti or opennms. They even have templates available for monitoring netapp systems.
If this is a Microsoft shop then consider using NetApp Appliance Watch module for MOM\SCOM as an alternative to Operations Manager, provided that monitoring is the only objective. If this is a Unix shop then consider configuring Syslog, but again only if monitoring is your objective. Operations Manager provides more than just monitoring.
For monitoring Netapps, I think LogicMonitor is better, in that it defaults to "monitor everything, all the time". i.e. point it at a NetApp, and all volumes are monitored for latency, usage, etc. All phsyical drives on all shelves are monitored for activity. All shelves are monitored. Add a volume - it's monitored. You dont have to enable latency monitoring for a volume, so you dont get issues where a new volume was added, didn't have latency monitoring enabled in Ops Manager, so you don't know if the DB performance issue was due to volume performance or something else. LogicMonitor will notice a new volume was added, or PAM card installed, and start monitoring it. (And uses the API, so it can get things like per volume ops and latency, fiber channel adapter utilization, etc, and other stuff not available via snmp)
It's also nice in that it can monitor everything else (windows, linux, databases, routers, etc).
Ops Manager is better as far as configuration and management of lots of NetApps, certainly, if you need to set up complex replications, etc.
I use home RRDs to monitor all our Filers : CPU usage, Net/iSCSI IOs, CIFS/NFS/FC ops, Disk Read ops / Write ops, disk usage, etc ... RRD is a very good OpenSource tool made for monitoring.
The only thing you have to worry about is getting the values of what you want to monitor; for that you can use SNMP or RSH. Everything else is taken care of by RRD : data storage, graphs, etc.
You can find a howto on building RRDs on my blog : http://bogdanochisor.blogspot.com/. For collecting data I use a Perl plugin from Nagios that has been adapted to read other OID entries. You can also modify the code to use "snmpget" or RSH commands (RSH is useful when values are not in the MIB, such as iSCSI Total Net IN or the number of packets per interface, etc).