Thwocker

Thwocker is an Atari 2600 game programmed by Charlie Heath that went unfinished and unreleased until its release in Activision Anthology in 2002. 
 

Gameplay 

The player controls a small man (or creature?) that does not stop rhythmically bouncing, though the player can control the height of the jumps. The object is to collect the correct musical note, displayed at the bottom of the screen, while avoiding floating musical instruments. Once the correct notes have been collected, a large note must be acquired near the top of the level. Grabbing that large note starts a song, during which the player can collect the remaining musical notes and instruments on screen for extra points.
 

Discovery

The existence of Thwocker was, at best, only assumed (the programmer was thought to have the only copy in existence, and no one knew what the gameplay was actually like), when a man discovered a blank, red label prototype Atari cartridge at a thrift store in California. The game within lacked a title screen and was only about 80% complete. He was only able to deduce its identity from handwritten scrawl on the cartridge: "C 1983 Activision Inc. TM Confidential 1/19/84 PR #001." How the cartridge ended up at the thrift store is unknown. A ROM of the game was quickly released to the web. 
 

Releases

The game was first officially released in 2002 as part of Activision Anthology, a collection of Activision-produced Atari 2600 games for the PlayStation 2.  It was included in Aspyr's Game Boy Advance port of Activision Anthology as well.  Most recently, the game was released for purchase on Microsoft's Game Room for the Xbox 360 and PC, costing $3. 
 
It should be noted that all three official releases of Thwocker are emulations of the same unfinished ROM rediscovered years ago.  The game has never been finished.