Do you just need monitoring or do you need control/automated operating system shutdown?

Network UPS Tools (NUT) is a really good option. Does USB, serial, and SNMP and covers a ton of UPS makes and models. You’ll find that a lot of SANs and other storage devices that “just work” when you connect a USB cable are probably using NUT under the hood.

If you just need SNMP monitoring, pretty much any opensource network management system could be made to work.

Observium is a good free tool for monitoring. It is one of those “just works” sort of monitoring systems that was easy to get started with. When I set it up I used the Turnkey Linux version , which makes provisioning even easier.

Nagios is really extensible.

Zabbix is a tool I’ve seen some of our customers using (although I think moreso with PDUs, but either should be achievable via SNMP).

LibreNMS is another opensource monitoring solution that is fairly popular.

There is a lot you can do with Net-SNMP and bash scripts too.

In a nutshell, if you have an SNMP-manageable UPS, whatever SNMP manager you use for your other devices can probably fit. Let me know if you need some more information (regardless of the UPS vendor you’re looking to monitor), this overlaps a bit with some topics I am looking into now.

edits: grammar, clarity

There is the fairly well-known free and opensource UPS monitoring tool NUT. It's available from the software repositories of major GNU/Linux and BSD distribution, including the OpenWRT distribution for routers.

I've used it in the past to log UPS data using simple shell scripts. The only requirement for using NUT is a communication port on the UPS, which could be a USB or an old-fashioned RS-232 cable. Most UPS systems with a communication port should work in dumb mode that would allow you to receive data from the UPS, if not send commands to it.

NUT is designed on a client/server model, where the interface is separate from the back-end.