There are many open source and free or inexpensive system monitoring tools available for Linux (and other open unices, ie freeBSD, openBSD, etc). I have tried several of these and none were quite up to the goals I had in mind, none until Zabbix that is. My goals for system monitoring are really very very simple. I want to monitor system parameters (processes, disk space, mem, etc), services (http. smtp, etc), particular applications (custom) and key files (checksums on key system files). I also want a system monitoring tool that is easily installed, easily setup, easily administered via a web interface, uses polling of agents for monitored machines and is capable of custom alerts and triggers for specified events. Thus far, again, Zabbix is up to, and in fact beyond, the task. Some background; I have tried Big Brother, NetSaint, Sysorb and several others - all of these worked to a degree, but had issues that I could not live with. Big Brother uses CGI to straight up "sh" shell scripts. Besides loathing CGI in general, the scripts didn't work on a Redhat 6.2 box that I was using for initial testing, I had to modify several things just to get it going. Once up it worked well, but has a clunky web interface. Sysorb worked well also, and was installed via RPMs (a nice convenience), but again required CGI (and the day after I got it working, it became commercial). More along the same lines happened to the others, except for Netsaint. Netsaint is a great program (except that it is discontinued and will forthcoming be Nogia, which does not have a release yet). Nevertheless, Netsaint was a bit more setup and maintenance than I wanted, everything has to be manually configured, and again, its CGI. So finally I stumbled across a few monitoring programs that used PHP by browsing at Freshmeat (and no PHP is not the same thing as CGI, as some will purport - but that's another discussion). Zabbix was one of these. Zabbix is a comprehensive monitoring system. Zabbix has a server and agents which are C and are compiled onto your platform (Linux, BSD, AIX, etc.) Zabbix requires a webserver for the frontend and uses PHP. Zabbix also requires a database, such as MySQL or PostGRES, but setup is very easy and took me only a few minutes. These are the only real prerequisites, webserver with PHP (and PHP needs GD graphics support compiled in IF you want to use another Zabbix feature, graphs and diagrams) and database. Quick Setup Instructions:

• 0. Create a user to run Zabbix binaries as (make sure to do this, this is step zero for a reason, dont run anything like this as root.)
• 1. Get the source and unpack it (see links.)
• 2. Setup your database (using supplied sql scripts.) Basically this involves creating a database and user for said database and running sql script on said database.
• 3. Make database settings changes to one source file.
• 4. Compile the server and agents using supplied configure script. (./configure - make.)
• 5. Change a few parameters in a supplied config.inc file for PHP database settings.
• 6. Move PHP files into a web location (somewhere in your docroot.)
• 7. Make zabbix entries into the /etc/services file (it uses ports 10000 and 10001, but note that this should not affect any firewall stuff because this should all be INTERNAL to your network.)
• 8. Run the SERVER on a specified server machine (zabbix_suckerd.)
• 9. Run the agent on specified agent machines (NOTE, the agent machines also require a file - /etc/zabbix/zabbix.conf - which contains the IP address of the server in order to ALLOW the server access to the agent.)
• 10. Login to the web interface and configure machines to monitor, what to monitor, when to monitor, triggers, alerts, graphs, diagrams, etc, etc, etc.

Thats it, it's very easy. Once all is running Zabbix also comes with a very comprehensive set of items already setup by default to monitor on each host. Zabbix can monitor normal stuff like system parameters (such as processor load, io, memory, ping, server uptime, etc), services (such as http, smtp, etc), and can ALSO monitor checksums on files, file sizes, inodes, and much more. One of the best things about Zabbix is that is also includes a very flexible and powerful syntax for specifying what should be monitored and alerts. With Zabbix you can setup a monitor on any process, file, port, etc that you want and set thresholds and simple logical statements about alerts. Zabbix keeps a history and stats on everything that is monitored and tracks trends. Of course it is all stored in the database (efficient, stable, extensible). And ,as alluded to earlier ,Zabbix also has the ability to display graphs and diagrams of hosts and networks along with pertinent stats. Its really really a very nice program that is simple, elegant and to the point. It is quite flexible in that it can do many complicated things if so configured, but it also can be setup to simply monitor very straightforward services. As far as security, it seems to be quite secure if run as a non privileged user, ports are not open to outside and the zabbix.conf file is required (and note that the web interface requires a login by default, but you should still place utilities like this in a protected directory so that configuration files, etc, cannot be accessed by unauthorized users). Zabbix is good stuff, checkit via the links (and note the screenshots) and thanks to its main developer Alexi.   http://zabbix.sourceforge.net/