Vincent Danen gives Nagios XI a big thumbs up for monitoring hosts and services -- the commercial version comes with a price tag but is worth it for the wizards that make setup a breeze.
I've looked at Nagios, the open source monitoring system, in the past and this time I'm focusing on its commercial counterpart: Nagios XI. Nagios XI is a product built upon Nagios Core (the open source offering) that really makes Nagios much easier to use and configure, due to its nice PHP front-end and some really slick sugarcoating.
Nagios XI does what Nagios Core does: it monitors hosts and services. Hosts can be almost anything: a router, remote Linux server, a remote Windows system, a web site. The services that can be monitored are impressive: ping responses, memory usage, CPU usage, bandwidth usage, whether a website has changed content, whether a particular service is available (e.g. SMTP, HTTP, DHCP, etc.) and a lot more. Nagios, both Core and XI, is impressive.
Nagios XI, as a commercial product, has a commercial price tag. If you're monitoring less than 50 hosts, it will set you back almost $1300USD for a perpetual license for the particular version of Nagios XI that you purchased (the current version is Nagios XI 2011). When a new version comes out, you would have to pony up for a perpetual license to that new version.While that seems like a lot, if you are depending on Nagios already or are looking for a monitoring solution for a lot of various hosts, compare the cost of setting up Nagios Core (not a trivial thing) to the amazing wizards that Nagios XI provides. Setting up hosts to monitor in Nagios XI is a snap. It comes with the Monitoring Wizard (Figure A), which has prepared setups to quickly setup monitoring for different services and hosts: use it to test email delivery, set up a Linux server to monitor remotely, check the response of a MySQL query or DNS query, the capabilities of an FTP server, SNMP traps, printers, arbitrary TCP or UDP ports... the list goes on.
These wizards are insanely simple to use. Take it from someone who has configured Nagios by hand and has dealt with the plethora of configuration files and configuration directives -- Nagios XI makes adding new services and hosts to monitor an absolute snap. If you have a lot of hosts to monitor, the time savings alone will make up for some of the cost of the product.
Nagios XI is available in two formats: a virtual machine appliance to download and deploy on VMware software (Player, Workstation, ESXi, vSphere) or as a source installer to be installed on a physical server; that physical server must run Red Hat Enterprise Linux or CentOS.
With Nagios XI you can enjoy a number of features you simply can't with Nagios Core. These features include performance graphs for each service, so you can see when resources might be peaking or when they are under-utilized - shown in very nice RRDtool graphs. Each host and service can be fully edited using the web interface. You never have to edit a text file -- everything is available through the web UI. There are neat ways to visualize data; I'm a really big fan of the "network replay", which visualizes the health of your network and how outages affect various systems, and the "alert heatmap" which shows a very cool representation of outages.
There are a variety of compelling reasons to look at Nagios XI over Nagios Core, beyond just aesthetics. The ease of configuration and maintenance is a huge bonus and well worth the investment. The additional reports are also quite nice (I don't know if I would buy a license just for the reports, but they are an enjoyable added bonus).
Nagios XI can be tried for 60 days without cost. Downloading the virtual machine image is a great way to test it out. Also, if you plan to monitor less than seven hosts, you can use Nagios XI for free. With the seven hosts comes an unlimited number of services, but with the free license comes no support (which you wouldn't have using the open source Nagios Core anyway). If you were using Nagios Core to monitor a small number of hosts, I highly recommend downloading Nagios XI and trying it out since you can use it without cost and take advantage of its many benefits.
I cannot recommend Nagios XI enough. I've been using it for a few months now and it is really a step beyond Nagios Core, which I had used for years prior. The support and development staff are very responsive and very helpful, which makes the idea of tossing down a good chunk of change a little more palatable. If you are using, or are looking to use, Nagios Core (or any other service monitoring solution for that matter), you owe it to yourself to check out Nagios XI.