The IOCCC (http://www.ioccc.org) is probably one of my favorite rabbit holes, since I program very frequently in C. I once fully explained one of the entries (2006-sykes2) on StackOverflow, which was quite educational for me: http://stackoverflow.com/questions/15393441/obfuscated-c-cod...

It's hard for me to pick a favorite, but perhaps one of the most memorable is 2004-gavare: http://www.ioccc.org/2004/gavare.c. This little gem implements a fairly sophisticated ray tracer using no C keywords or preprocessor directives at all: no "int", "char", "if", "return", "#define", etc. I learned a great deal about computer graphics (and, yes, C programming) from reverse-engineering that program.