I noticed we didn't have a thread to discuss unreleased games, so this thread is for unreleased, canceled and prototype games, starting with the recent reveal of Hardcore:
DICE unveiled Hardcore at the 2010 Datastorm Party. Hardcore is an unreleased Sega Genesis/Mega Drive game that appears to be a difficult Turrican clone.
The game was being developed for Psygnosis in 1994 and was 99% completed (reportedly there was only one bug left to squash) when Psygnosis suddenly canceled all future Mega Drive titles. DICE has been trying to release the game into the public domain, but Sony still owns the rights to Hardcore.
Watching the gameplay video for the first time really excited me. It's been ages since I've seen a new Genesis game, and everything about Hardcore is deliciously old school, from the difficult gameplay to the visuals to the Amiga/MD style chiptunes, which set my heart aflutter upon hearing them. Hopefully DICE can untangle the licensing issue and finally release this game.
Interestingly, a related video for Hardcore brings up Bill's Tomato Game, which was released on the Amiga. It seems that a Mega Drive version was in development by Psygnosis but suffered the same fate as Hardcore and was ultimately canceled.