The people at Hidden Palace have found a couple of months ago, quite possibly one of the most infamous cancellations in gaming history: 100 Bullets by Acclaim Entertainment, based on the Vertigo comic of the same name. This game was notable for having been cancelled at the last hour, when Acclaim closed its doors in late 2004.
The April and August 2004 prototypes were built for the Xbox whereas the June 2004 version was buit for the PS2, which includes levels seen in the E3 2004 demo.
The D3 Publisher version of 100 Bullets, who bought the rights of the comic after Acclaim went bankrupt, was also cancelled and it's yet to be found.