obfuscated assembly language
Assembly language without comments or symbol names. This is what all programs eventually end up as. But why let a compiler have all the fun? Just open up your favorite boot monitor or low-level debugger and start tapping away (on the numeric keypad for the really ambitious) a new kernel or window manager or anything else.
Well, yes, there is the practical side of it all...
The first assembler I learned was x86. Turned out to be good for optimization sometimes, but not really that useful in the long run. Intel has so many instructions just for backward compatibility that it's quite a pain to master and it is quite a challenge to write readable code.
But, assembly language for the PIC microcontroller is a whole different story. Because of the PIC's very limited memory assembler is the only way to go. Because most of the PIC series controllers have less than 40 instructions, it's very readable and easy to program. Obfuscating PIC assembler is more of a challenge, when in x86 assembler it's a way of life.