Here's a great list of network monitoring tools:-
- Directory of monitoring software
- Directory of monitoring software
- Directory of monitoring software
- Directory of snmp monitoring software
- Freshmeat network monitoring software
- LNM-3.5 Network monitoring bundle with Asterisk included (Dutch)
- Zetamonitor Network monitoring in the cloud for servers (with preconfigured agents) and network devices with SNMP support
I use http://www.zetamonitor.com services for keep monitored my MS SQL Server and Cisco Catalyst switches and it works very well as I don't have to install nothing on my network.