I second Nagios if you've got another server to run it on. It's got this great client called NRPE that can run tons of plugins. I've got it setup on quiet a few servers with SMS notifications monitoring disk usage, processes, exim queue size, latency, etc.
OpManager is the best thing smokin' in my book. However to use this great product it will cost you $795 for the professional edition. If you have 25 or less devices to monitor you can use the professional verision for free.
see Argus. Free, open-source, clear browser-based display and management, logical configuration file format. Very reliable. I have used it for at least 3 years now to managed thousands of services and have only been impressed.
George Donnelly / Systems Administrator
High Speed Rails inc. / FOSS Hosting http://highspeedrails.com
"Linux is Luke. FreeBSD is Yoda."