I find undocumented opcodes most interesting. They were literally holes in the 6502 circuitry. People used them on the C64 to get more speed out of their assembly code. Early C64 emulators couldn't cope with those games/demos. Luckily, they are documented in disk magazines, etc, so they've been implemented now.
byuu is also doing this for the SNES with bsnes - he's even had all of the enhancement chips like the DSP-1~4 and CX4 decapped to achieve cycle-perfect emulation.
Well, the decapped chips have been photographed, but they haven't been made into working diagrams like that 6502 one.
The important thing about the decapped chips is that any on-board ROM has been dumped (electrically, not visually) so that they can be emulated with an opcode interpreter, just like other emulators.
