3 monitoring programs in one????

Well not really.  This article does a good job of explaining the difference between Nagios, Icinga and Opsview.  At the core these are all Nagios though.  Missing from this set is Groundwork Opensource which is also based on Nagios, which we reviewed last year.  If you haven't looked at the original or these two alternatives give the article a look.  If you are evaluating Network Monitoring/Datacenter Monitoring solutions to check out Zenoss, we we also reviewed last year.  While inspired by Nagios it is a completely new process.  But as a testiment to how awesome Nagios is still can use any Nagios plugins that you just can't live without.