I was bored one day, so I decided to write a full-blown *NIX-like shell for Windows in NT Batch script. Four months later, this monstrosity ended up containing over 50 *NIX command look-alikes. It works on Windows XP and newer.
However, it’s too glitchy and full of licensing conflicts to be released publicly, not to mention the existence of Cygwin and MinGW/MSYS. Still, it was a very nice learning experience.