In video gaming - and the technology world in general - there's a term for hardware and software that's promoted and promised without a product even hitting the market: Vaporware.
Sometimes, years later, dreams of the past are dramatically exhumed and become reality, Duke Nukem Forever being easily the most notable example. Generally speaking, though, when it comes to the biggest pieces of vaporware in gaming, unreleased consoles are usually what come to mind. Sony, Sega, Nintendo, and Atari have all gotten heavily into development for new consoles that never hit the market, as have mainstream electronics companies like Panasonic and a whole host of companies you've never heard of.
Some of these actually got pretty far, with announced release dates, games getting previewed in many of the popular magazines at the time, and all sorts of other hype. At least one of these consoles completely changed the face of the video game industry. If this console was actually released, it's entirely possible we'd have at least one fewer player in the current console market.
On that note, it's time to look at those consoles that just vanished into the ether.