Ask HN: Best Open-Source Monitoring?

I maintain a bunch of small things: little web sites, a Twitter bot, a couple of home servers, etc. Currently I'm getting rid of my colocated server and moving a bunch of stuff to the cloud. (Yay terraform!) 7 years ago, I ended up with Nagios for monitoring and alerting, but I've never really liked it. What are folks using these days? What do you like about it?