I've known of tons of illegal moves used in boxing. I'll just mention one here for sake of humor.

Let's say for instance that your opponent has been making sure that he's well hydrated, upon noticing it you go for the other low blow... right in the bladder! If you don't know the bladder is just underneath the beltline. Anyways a hard well timed shot should make them piss themselves, or the worst case scenario it'll rupture their bladder killing them in one of the most embarrassing and unforgiving ways possible. I'd save this for boxing's reprobates right next to avenging your best friend's death. However it's not unexcusable to throw it in as a little lovetap for that buddy of yours that keeps cheap shot'ing ya.