Nagios, what is likely one of the most useful and popular applications out there today, is something my company wouldn't be able to survive without.
It's is a free open source computer system and network monitoring software application. It watches hosts and services, and makes for a very powerful alerting tool, not unlike a nagging mother-in-law that's always right. Its name is a recursive acronym for 'Nagios Ain't Gonna Insist On Sainthood', sainthood being a reference to the original name of the software, which was initially called NetSaint upon its creation back in 1999. Earlier this year, in fact, Linux.com did a Just for Fun Poll, asking Linux users what their favorite IT operations tool was. Nagios came out on top with more than fifty percent of the vote.
It's understandable why so many people love the tool, as it is free, powerful and flexible. Administrators at companies large and small are generally faced with a multitude of hardware and software to pay attention to, and find themselves incapable of physically monitoring each specific item. Thus, Nagios is a godsend (read: a saint perhaps?) to these admins, by keeping track of all the services in the infrastructure, raising alerts before small issues become large ones, provide a look into the entire system's status, etc. And, although the system can take a moderate amount of configuration, it does this all for free – my favorite price.