We should have a best unfinished game contest!!
OK, I'm entering Iron Fist which can be found near the bottom of this page:http://www.greyaliengames.com/games.php
You can also see that Jewel of Orion isn't finished either.
I got DOS version of jewel or orion too that was never finished in C++ years ago. Also an unfinished platform game made in C++.
Going back further than that, there would be ooooh a ton of Blitz Basic 2 and Assembly games on the Amiga. Then before that Spec, BBC, C64 etc...