These codes are all formatted to work with the Action Replay. You'll need one of the older versions that actually let you input custom codes; I've heard that the newer versions don't allow this.
Also, any of these codes can be used in the Dolphin Emulator... but you'd rather be playing on a real 'cube, right?!
*All codes were made by me, Trebla Yevrah.
Biohazard 0: Trial Edition
Tony Hawk's Underground
Most of these images showcase the "room modifier" in action. Practically every area exists on this single demo version disc, albeit in early-extremely rough stages.
On an actual Gamecube, many of the doors around the training facility can crash the game, but some are fully functional.
No enemy placements are enabled beyond the train. Some puzzles are at least partially operational, but nothing is what you'd call "finished." The trial edition is obviously from a much earlier build of Resident Evil 0.
The famous "lost levels" of THUG were discovered years ago for the PS2 version. Was it difficult to find them on Gamecube? Not really... I guess nobody cared, that's all!
Just to give you a rundown of what these are, they're just three classic stages that Neversoft originally intended to include. Due to time constraints, I suppose, they had to be scrapped. They would eventually be finished for the sequel, Tony Hawk's Underground 2.