As mentioned before, Game Zero is the project name for a game in development by Zoonami. Zoonami was founded in 1999 by Martin Hollis, one of the bigger guys at Rare during the SNES/N64 days. They still haven't released much info, but it started pre-production in 2000, which means that if they haven't totally redesigned the game multiple times, they should have something to show sooner rather than later. Of course, Zoonami seems to be pretty small and likely has some of the Rare 'it's-done-when-it's-done' mentality, so the wait might be even longer. There was no sign of it at E3 which makes me think the game won't be finished for another year.
As for a couple of the other games mentioned in the thread, Mini Racers was supposedly completed for the N64 but never released. The original dev team disbanded (Looking Glass Studios) so they wouldn't have been around to update it for the Cube. Nintendo was supposed to be in possession of the code, so maybe parts of it have gone into other games...or maybe it wasn't quite up to Nintendo's standards and put it to rest.
Buggie Boogie was turned into a solid demo by Angel Studios that helped them land a deal with a publisher. Seeing that Angel Studios is now Rockstar San Diego, I'm pretty sure the demo eventually led to what would become Smuggler's Run (a game developed by Angel Studios and published by Rockstar).
As for Rev Limit, Falcon, and Stunt Racer...I have no clue (I don't even really remember hearing about Falcon). The Rare Gran Turismo killer was a rumour that was never confirmed. Any number of things could have happened to it if it ever existed. Keep an eye out for Rare's future Xbox games and maybe they'll give us some ideas. That applies even more so to Donkey Kong Racing. I'm fairly certain that's getting a facelift and will show up on the Xbox as some point.