Being able to have several channels on TV at the same time, in tiles. I'm not clear on why we don't have this yet. You could do something similar on a computer with a bit of effort and processing power, but I'd like to do it on a real TV, like in the movie.
Also, Mr. Fusion. I think about that one all the time. There would no longer be any such thing as garbage--it would all be fuel. :)
Hover boards actually were created, if I recall correctly, but they're not mainstream. The standard board would still be more preferable since you can actually feel it. Power laces are also invented but why buy a shoe for who knows how much cash simply because it has power braces when you could buy the same shoe for less than half the price and just tie it yourself? Clothes that dry themselves are a thing as well. But, again, not mainstream.