Zabbix is released under the GPL license, thus is free for commercial and non-commercial use.
There are no limitations on the number of monitored devices, you can use Zabbix to monitor many thousands of devices absolutely free.
You can modify the source code to suit your system and use it internally, as well as develop customized tools aggregating features to Zabbix.
No Vendor Lock-in
The source code is fully available, so the IT environment of your company will not become dependent on a commercial entity.
All settings and the values collected are stored in a simple format and are fully open, easy to export or integrate with other systems.