I have read quite a few articles in various mechanics and car magazines and none of them really work. I think a few things work to a small degree but typically are not worth the extra trouble and expense. An example would be methanol/water injectors. I think they work but you probably would end up spending more for it than it would save.
I did this to a Ford Pinto station wagon in the 70's. All it takes is a container rigged so that the vacuum pulls the liquid from the bottom of a gallon jug. with a few other changes, I did realize about a 4 mpg improvement. I can't remember how I arranged for the drip. The whole thing couldn't have cost me more than about ten bucks. It definitely needed a hydrocarbon liquid mixed with the water to make it work. At the time I switched to a small molecule triple refined oil and changed the carburator jets. The original set up on the engine amounted to a detuning. I think I also needed to advance the spark. It was a long time ago when engines were much easier to get into. I can't remember all the details but it was fun trying to squeeze a little more out of each gallon.