We've seen quite a few retro games get the decompilation treatment recently, but there's a new work-in-progress project that is attempting to do something a little different.
The process of decompilation basically turns 'executable' code back into human-readable source code, and zeldaret is doing just that with the N64 emulator used in the GameCube release of The Legend of Zelda: Ocarina of Time.
Released in 2002, the game came bundled with Ocarina of Time Master Quest, a version with increased difficulty that began life as a 64DD title.
This is quite an unusual decompilation project, as unofficial N64 emulators exist that are arguably more advanced than the one Nintendo used back in 2002, but it's interesting from a technical perspective, at least.