HHO is one of the many monikers for Brown's Gas.
They aren't using Hydrogen tanks. They are using electrolysis to create Brown's Gas which is fed into the engine and is expected to improve fuel mileage. The extra weight is a gallon or less of water. $50 doesn't buy you the kit, it buys you some information and you gather the kit yourself. "Run your car on water" is overstating what it does. The car still runs on petrol, just less.
There's good science and claims that says it doesn't work and there's claims that says it does.
There's lots of places that sell this stuff and there's lots of discussion about it, here on BITOG and other places. Read up.