ReactOS, the effort to create a free and open source Windows NT reimplementation.
It has been in existence in some form or another for nearly 30 years, but did not gain the traction it needed and as of writing it's still not in a usable state on real hardware. It's not abandoned, but progress on it is moving so slow that I doubt we'll ever see it be released in a state that's useful for real users.
It's too bad, because a drop in Windows replacement would be nice for all the people losing Windows 10 support right now.
On the other hand, I think people underestimate the difficulty involved in the project and compare it unfavorably to Linux, BSD, etc. Unix and its source code was pretty well publicly documented and understood for decades before those projects started, nothing like that ever really existed for Windows.
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