I have ADHD and I now tell people that I think a lower carb diet is the first line of defense. I'm also personally a big fan of medication, but for mild cases diet alone might be the main management tool someone needs. I think for some situations with ADHD that you don't need to go very low carb. Maybe to start. Then people might happily settle in at around 75–100g/day in combination with a Paleo/primal style diet. You want to avoid wheat and junk food.

I've also heard some excellent reports of people controlling bipolar disorder with very low carb diets.