Add Menuet, the OS written in assembly, to that list too:

http://www.menuetos.net/

There's also LoseThos (http://www.losethos.com/), although it's a bit on the eccentric side.

Don't forget http://www.loper-os.org/

by far my favourite.

Hactar is "Possibly, an Operating System": http://anynowhere.com/bb/posts.php?t=410