I used Nagios for health/performance monitoring of devices/servers for years at a previous job. It has been a while, and I'm starting to look into this space again. There are a lot more options out there for remote monitoring these days.
Here is what I have found that look good:
Do you know of any others I am missing? I'll update this list if I get replies. The requirement is that there must be an Open Source version of the tool.