6502 machine language
6502 machine language is a low-level programming language that maps 1-to-1 to the instruction set of the MOS 6502 microprocessor. Programmers usually write their code in 6502 assembly and then compile it into 6502 machine language to run on the chip, however, if you're working with compiled programs, you usually have to work directly in the machine language.
In my desire to hack NES games more effectively, I started delving into 6502 machine language. While I am by no means a competent 6502 programmer, I know enough about the chip to perform basic hacks to existing programs. I did a fair amount of work writing about the language in my NES Hacker Wiki.