Urine- pig urine, your mom's urine, your own urine. I have heard this is the natural remedy because urine has tons of good bacteria (needed to keep the urinary tract clean)- Good heavens, don't take my word on, though ;0)
I've heard this quite a bit, too. Even from folks who use this method quite religiously and swear by it.
Urine is also supposed to be great for jellyfish stings.