Somewhat less obscure operating systems I didn't see mentioned...
DragonflyBSD
Bitrig (
OpenBSD fork)
GNU hurd
Linux is Linux is Linux...what's different between different distros is pretty minimal because they all start with the Linux kernel + GNU. The major differences seems to be the package installers and the syntax to add/remove a package is not a big differentiator to me. If you want something different for Linux, try
http://www.linuxfromscratch.org/