NiL (recursive acronym for NiL Isn't Liero) is a clone of Liero, which runs on Linux and Windows and is released under the terms of the GNU General Public License.
Xinu ("Xinu Is Not Unix", a recursive acronym) is a Unix-like operating system originally developed by Douglas Comer for instructional purposes at Purdue University in the 1980s.
EINE, (a recursive acronym standing for "EINE Is Not Emacs") - an early Emacs text editor for lisp machines