The Brainfuck Programming Language. Brainfuck is the ungodly creation of Urban Müller, whose goal was apparently to create a Turing-complete language for which he could write the smallest compiler ever, for the Amiga OS 2.0.
His compiler was 240 bytes in size. (Though he improved upon this later -- he informed me at one point that he had managed to bring it under 200 bytes.) I originally started playing around with Brainfuck because of my own interest in writing very small programs for x86 Linux. I also used it as a vehicle for writing a program that created ELF files.