Monitoring the IT infrastructures of both our company and our clients as well is a central part of our job. For this reason we use a variety of monitoring tools depending on each specific type of activity. A relevant (to us) usage of these tools is to achieve a global vision of all the infrastructures that we care about, irrespectively of their location and their identity. These infrastructures can be accessed (and maybe controlled) by their respectiv owners, who can perform actions that don't have to interfere with ours.
This scenario was implemented through a series of standalone monitoring stations that had the Check-MK Multisite software: each station was independent but at the same time all the data was available to us. Check-MK Multisite allows to organize an interface to Nagios (and compatible systems) where the monitoring data from each station are visualized in a single dashboard, through the mk_livestatus protocol.
The dashboard allows also to execute Nagios commands on the monitoring stations, allowing to centralize the management of each station and to leave them independent at the same time.