There were plenty of those, including commercial ones.
It's pretty hard to find but ~25 years ago I was using Xi Graphics Accelerated-X which had 3D acceleration long before Xfree86.
Update: but yes I imagine it had some code from original MIT release.
For completely independent one you can have a look at WeirdX/WiredX, which was written in Java and even supported antialiasing and transparency for core protocol (something that Xfree86 people claimed to be impossible to implement).
Please cite a single independent implementation of an X11 protocol server.