Assembly language or ASM
Assembler is one of those languages that typically strikes fear into most people. Many say the day of ASM is over, and they of course couldn't be more wrong. Learning ASM will typically lead you to a far more fundamental understanding of how your computer works, and that all important memory management. Put it this way, you simply can't go wrong by investing some time in at least learning the basics of ASM. I for one would strongly recommend it.