Uptime Kuma for web monitoring.
I'm experimenting with both Zabbix and Netdata to see which one I want to keep for monitoring resources on my hosts.
I use healthchecks.io to monitor backup scripts and cronjobs.
I'm using Autoheal to restart containers that are in an unhealthy state. For some containers this means I need to write my own health check. I mostly did this to resolve a rare issue where Plex would lock up but it's helped in other scenarios too.