Nagios used to be only Open Source then they created the Enterprise version and left the open source core version lagging behind, it was forked a billion times

Nagios used to be only Open Source then they created the Enterprise version and left the open source core version lagging behind, it was forked a billion times or more :) creating the Nagios Effect. A lot of monitoring software / companies then removed / replaced the core of Nagios from their products.