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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Linux is a family of open source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991 by Linus Torvalds. Linux is typically packaged in a Linux distribution (or distro for short).
Distributions include the Linux kernel and supporting system software and libraries, many of which are provided by the GNU Project. Many Linux distributions use the word "Linux" in their name, but the Free Software Foundation uses the name GNU/Linux to emphasize the importance of GNU software, causing some controversy.
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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.
Canoeboot is more of a sister to libreboot than a replacement
https://libreboot.org/news/canoeboot.html
Thanks for the correction!