The First 15 Minutes of Half-Life (Dreamcast)

Copied from Sega Retro:

Half-Life is an unreleased Sega Dreamcast first-person shooter developed by Captivation Digital Laboratories and Gearbox Software. The highly-anticipated port of Valve Software's popular Half-Life, it was originally scheduled to be published Sierra On-Line in late 2000 - a date which would eventually be pushed back to June 2001. With Sega's official discontinuing of the Dreamcast in March 2001, Sierra On-Line announced the port's cancellation on July 31, 2001, citing "changing market conditions".

By then, Half-Life's Dreamcast version had already received a significant amount of both advertising and public awareness. Virtually every part of the game's production had been completed, from promotional kiosks to magazine reviews, and its official strategy guide had even been shipped to a small number of retailers for sale to consumers. With such a high-profile title seeing a very late cancellation, Half-Life has since developed a reputation as one of the Dreamcast's most infamous unreleased games.