There exists a small, but very dedicated group of people who hunt down images, information, and most importantly working prototypes of video games that were never released. They're kind of like video game archaeologists. Their entire goal is to find these games, preserve them, and release working images onto the net so everyone can have a chance to see or play them. Thier arch-enemies, of course, are so-called "rom hoarders", who collect these games and refuse to allow them to be archived or preserved because that would mean they wouldn't be the only people on the face of the earth who can play these games. Sad, retarded, but true.
Please keep in mind that we're not talking about rare games that had a small release, these are games that only a smattering of information is known about them via a few screenshots from magazines, rumors, and less. As you can imagine, the legends around some of these prototypes have reached epic proportions. Believe it or not, some of these have been found at flea markets, pawn shops, or on Craigslist being sold by people who don't know what they have.
Strider (Supergrafx)This is the Lost Ark of prototypes. Two screenshots are known to exist -- a comparison with an arcade screenshot was made, which shows they are too different to be the same game. One person claims that exactly five prototypes were made, four forever locked away and one that may or may not be in a collector's hands. The story behind this game could be a novel - one rumor claims that the programmer committed suicide, another claims he is hiding in Kobe somewhere. One of EGM's editors has claimed to have played the prototype in Japan. Japanese game magazines gave conflicting reports on its development.
Bio Force Ape (NES)A few screenshots from Nintendo Power exist. Subject to several hoax prototype "releases". Now a bad joke in the ROM collecting community. Mentioning it on any of their boards will probably get you banned.
Dracula X (Sega CD)Announced by Konami to be in development. Some years back Konami posted an image of some sprites that never made it into any game, some speculate that it was from this. Nobody knows.
Final Fantasy IV (NES)Several screenshots were shown in a magazine of the development of the fourth Final Fantasy for the Nintendo Entertainment System. Not the SNES. The sprites looked similar to the overworld characters from the first Final Fantasy with nicer town graphics. Scrapped, as we all know, and FF IV proper was released on the SNES. It is unknown if a prototype or even the code to this game still exists.
Hard Drivin' (NES) Screenshots of this game exist. Reputable accounts of seeing this game actually running on the NES hardware also exist. Polygons. On the NES. Damn.
Symphony of the Night (Game.Com) A port of the PSX SotN to the crappy Tiger Game.com. Screenshots were shown, but personally I'm willing to bet they were just PSX screenshots cut down to black and white.
Hellraiser (NES) Heavily advertised for a company (Color Dreams) that could find few retail outlets for their games. This game was supposedly going to have an extra on-board chip (a Z80, I think) that would enable the game to deliver a 16-bit like experience. Not much is known. Only some mock-up graphics are known to exist.
More to follow
Mother (Earthbound) (NES) Earthbound may have not been the biggest hit on the SNES, but its fans have probably the most annoying presence ever. Most video game forums can't go a month without some douchebag posting a thread about this stupid game. Earthbound was actually a sequel to a Famicom game known as "Mother". A prototype was uncovered with a translation that Nintendo never released. As you can imagine, the Earthbound fans jizzed everywhere.
Good Luck, Charlie Brown (Atari 2600) Long rumored to exist, found in a flea market. The game is about Charlie Brown vs the Kite-Eating Tree. Not yet released as a rom image.
Lord of the Rings (Atari 2600) Another game fueled by speculation, it was featured in a catalogue in 1983 and turned up in 2001 and is surprisingly complete. The image was released on the internet for download.
Penn and Teller (Sega CD) Completed. Became famous for the "Desert Bus" minigame which became a successful charity. Also, it's a game about Penn and Teller.
Sunman In the same vein as Sunsoft's late Batman release for the NES, it was supposed to be a Superman game. Never released. It did some pretty incredible things with the graphics like multi-scrolling backgrounds and the like.
Sonic Xtreme This was supposed to be the long-awaited Sonic game for the Saturn. Never released, a prototype disc was sold on ebay to a zealous Sonic fan for $2500 so he could have it all to himself. Another copy was found and soon after released for free on the internet. It would have been worth it to see the look on that fan's face when his purchase became effectively worthless.
Super Mario Wacky Worlds (Philips CDi) One part SMW sprites, three parts ugly-ass fan game. Never released. A terror to behold.
StarFox 2 (SNES) If I remember correctly, this game was actually stolen from an electronics show or something where Nintendo was showing it off and years later the rom was dumped, and fixed to be made playable on emulators. Almost completed. Probably the most epic find so far.
Kid Icarus (SNES) This was actually a homebrew by YOSHi. From what I remember, it's a title screen and nothing more. Fooled a lot of hopeful people.
Zelda III (NES) A rom hack that was set up to look like it was running on a NES and playing on a TV. Fueled a lot of speculation, anger, and butthurt.
Final Fantasy VI (N64) This was a tech demo Square made that a gaming magazine falsely reported as running on N64 hardware. In reality, it was running on an actual SGI system, it was a quick 3D demo using characters from Final Fantasy VI, and it was never anything more than that. Mentioning this is known to still cause Nintendo adherents to break into a crying fit.
Pirate Carts (Crazy Catch-All) Ever wanted to play Super Mario Brothers 19, Contra 8, Sonic in the Mushroom Kingdom, and King of Fighters 32? You can, if you buy those crazy pirate carts from east Asia. Most pirated games that are marketed as a "sequel" are just a simple hack of the same old game you know (and are bored of.) Some pirated games, however, are surprisingly sophisticated and have fooled a lot of hopeful rom collectors.
The Valis anime Back in the early 90's, right before Usenet stopped being the middle ground for discussion and became the horrible crack alley it is today, there was a lot of speculation on whether an anime about Valis (the PC-Engine CD game series, not the Philip K. Dick novel) actually existed. Some claim to have seen it, others swore up and down it simply did not exist. As it turns out, there was a 3-minute animated promo made for the Famicom version, and it's now on Youtube. That is, if you care. And you probably don't.