Hurd seems usable ( this video shows many interesting features http://archive.org/details/SamuelThibaultOnGnuHurd )

With the "let's rebuild everything" trend I hope Hurd could be brought to light again. Just to shook the view of what an OS is, and how its design influences the upper stacks.

BTW, how many single-lang userspace projects exists ? perl, lua, any other ? lisp ?