μMon: Stupid simple monitoring (2022)
I like the concept of simple monitoring. Simple means it is simple to install, simple to maintain and simple to use. 
For me, this is netdata. Netdata could be much more, but I just install it on whatever machine and never think about it again. And when something is strange on that machine, I go to <a href="http://localhost:19999" rel="nofollow noreferrer">http://localhost:19999</a> and look around.