Half-Life (found Dreamcast port of first-person shooter; 2000-2001)
Half-Life was a first-person shooter developed for PC by Valve and originally published by Sierra Studios in 1998. The game has received nearly universal acclaim from both critics and consumers and is often regarded as one of the most revolutionary games ever created. In the early 2000's, there were plans to port the game to both PlayStation and Dreamcast. While the PlayStation port was eventually released, the Dreamcast port never saw an official release.
Gearbox and Captivation Digital Laboratories were slated to develop the port, with Gearbox creating content exclusive for the port and Captivation handling the actual porting of the game. Aside from graphics improvements with models having twice as many polygons, there was also an exclusive bonus mission planned called Half-Life: Blue Shift, which would eventually find its way to the original game as a whole expansion pack.
The port had several delays after sending out review copies and getting very poor reception from critics. A few weeks before it would have officially released, despite the fact that it was finished, the port was cancelled outright in combination of the poor reception and "changing market conditions" with the Dreamcast. At the time of this announcement, there was even a strategy guide that had been printed and ready for publication.
A few years after its cancellation, a late build was discovered. The given build number was build 1672, and it was dated May 2001. The port is fully functional, and can be played on real hardware, proving that the game was effectively complete when it was cancelled. The ROM dump can easily be found online.
Additionally, in 2012, a mod was released for the PC version of Half-Life that ported the content from the Dreamcast version.