Macs Fan Control is not entirely open source, but the developer (CrystalIDEA Software) does maintain the source code on GitHub, where users can track issue reports, participate in translation efforts, and follow development progress.
Most users seek a license key to solve specific hardware or workflow issues: Monitor and control fans on Apple computers - CrystalIDEA mac fan control pro license key
Purchasing Pro directly supports the continued development and updates of the software. Macs Fan Control is not entirely open source,