Being able to query a server locally and find out what it's doing is great, but that's rarely how things are done in the real world (outside your single box that you might maintain for personal projects). In company scenarios, it's much more likely that you'll have a monitoring solution of some sort, perhaps with agents on your boxes, which keeps an eye on the health of machines in your care.
Nagios is the undisputed king of monitoring installations the world over, not because it's the best, or the most flashy, but simply because it's one of the oldest, and once a monitoring solution is in place, you'll find teams are very hesitant about switching over to a new one.
It has caused several clones to be created, and various offshoots (some using the original source code and some not), but all of them will behave in a similar...