The best open source alternative to Domotz is Nagios, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 25 alternatives to Domotz and five of them is open source so hopefully you can find a suitable replacement. Other interesting open source alternatives to Domotz are Port Authority (Free, Open Source), Cabot (Free, Open Source), WinAudit (Free, Open Source) and NetXMS (Free, Open Source).
A handy systems and security-focused tool, Port Authority is a very fast Android port scanner. Port Authority also allows you to quickly discover hosts on your network and will display useful network information about your device and other hosts.
WinAudit is an inventory utility for Windows computers. It creates a comprehensive report on a machine's configuration, hardware and software. WinAudit is free, open source and can be used or distributed by anyone.