As with anything, each tool has it’s own set of strengths and weaknesses. Some are the applications may seem similar, but are very different and range from full blown SNMP management solutions, to simple applications with not much flexibility. Big Brother, OpenNMS, OpenView and SysMon (there are dozens more) are often compared to Nagios, however they are quite different in many respects. In my travels as an IT professional, Nagios is the most commonly used monitoring tool by far. There are lots of specialist companies that offer monitoring as a service. A large number of these use Nagios.