A pseudofree implementation of an X11 server, as implemented by the X consortium for 386-class PCs. It never kept up with features and was buggy and slow as all get-out, and a couple of people wanted to fix all its problems and improve it but the X consortium decided to be bastards and not accept outside patches.

And so XFree86 was born (its name is a pun on X386, and never really had anything to do with the architecture it ran on). Originally it was based heavily on the X386 code, but it's been completely rewritten from scratch as of XFree 4.0.