Nagios is probably the best known open source network management tool. Ethan Galstad created NetSaint, the tool that eventually became Nagios, many years ago at the very dawn of using open source tools in network management.
Things are not going well. A number of people from the Nagios community, including a couple from the Nagios Community Advisory Board have decided to create a fork of Nagios under the ICINGA project. The reason? The founders of the ICINGA project charge Nagios with stifling the development of the project.
It will be interesting to see how much traction ICINGA attains. Forks often fail to gain the necessary momentum to be successful.