Open source software has had a foothold in the enterprise monitoring sphere for almost as long as open source has existed. One only needs to look at the sheer ubiquity of small applications such as MRTG (Multi Router Traffic Grapher) and its RRDTool back end to see that. What we haven't had from open source is the big application -- the comprehensive, community supported open source enterprise management suite that provides the depth and breadth of functionality that businesses need and generally find in closed-source competitors. That is changing in leaps and bounds. In fact, open source enterprise monitoring solutions are evolving so quickly, we won't even try to declare a clear winner yet -- but we're working on it.
Not all that long ago, an entirely open source monitoring system would encompass software from five or six different open source projects. A typical configuration might include the very popular Nagios for monitoring, one of many available configuration front ends to make Nagios easier to manage, a plethora of Nagios plug-ins, a package such as Cacti for more advanced trending and graphing, and a few Cacti plug-ins such as WeatherMap and Thold. Experience indicates these packages together form an extremely powerful and infinitely malleable solution.
By the time the entire collection has been fully configured and integrated, however, someone has spent a significant amount of time getting it completely dialed in. Any addition of new resources to be monitored usually requires fairly intimate knowledge of how things were assembled to begin with. Worse still, upgrades can be complicated due to the interdependency of various components. None of this would seem particularly arduous to a reasonably experienced Linux/BSD user, but it can be downright horrifying for someone without Linux or Unix experience. That fact alone has prevented open source monitoring from penetrating the average, largely Microsoft-based medium business.
All of that is starting to change. Within the past several years, major new, commercially backed, open source monitoring packages have been arriving on the scene at a dizzying rate. Some of these packages provide the glue necessary to coherently unite disparate packages that hadn’t previously been integrated. Others have been grown from the ground up, their backers aiming at providing a completely unified solution. Major players in this space include GroundWork, Hyperic, and Zenoss.