You can run a free OS pretty effortless, but when wanting 100% free software, you have to dig deeper and replace the proprietary BIOS firmware.
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Besides the already mentioned Star Labs and System76, there’s also Insurgo, Nitropad and NovaCustom.
As for an exhaustive list on the matter, unfortunately, I don’t think something like that is out there. Though both Canoeboot (formerly known as Libreboot) and Dasharo do have their own respective lists.