LibreNMS. it works great and it's free.
networking
Community for discussing enterprise networks and the ensuing chaos that comes after inheriting or building one.
Zabbix, is pretty good monitoring software that can be used on prem. It has most of the features of solarwinds in its agent and SNMP monitoring but the default UI and integration isn’t a nice as Orion.
If you are looking into cloud based, we use NinjaOne which is RMM platform but requires a endpoint to install its NMS extension to do SNMP monitoring. It comes with a whole lot of other features and integrations such as remote access via teamviewer/Splashtop, backup, and documentation modules, etc
LibreNMS. it works great and it's free.
Another vote for LibreNMS. I've been using it for a long time and it's just great for most small - relatively large orgs (you have to work a bit harder to deploy it properly / distributed, if you're going for a larger build).
I've also had Zabbix data piped into grafana and that was rock solid.... I just find that Zabbix requires quite a bit more finessing to get going, if you're not a seasoned sysadmin.
I’ve been using Intermapper for years now. It has its quirks but it fits my needs.