Five Below is another "dollar store" in that blend, focusing mostly on goods for young adults and teens. Originally everything was below $5, up to and including some video games and electronics like computer accessories or even drones.
It isn't too new of a niche, closeout retailers have often existed and its not uncommon with small businesses; ocean state job lot in the NE kind of exists with this model. I think what is unique is the small retail store model; dollar generals are as big as a walgreens or CVS, but stock a pretty full assortment of goods for the size. They are pretty impressive for being micro-super walmarts in a sense.
Dollar tree's are interesting because you can actually find books and dvd/blu-ray there. It's sort of a commentary on how tough it must be to be a creative when I can buy remnant books from A list authors sometimes; I've seen Jonathan Franzen, John Scalzi, Bill Mckibben, And Rod Dreher books end up in the dollar bin. There's also a staggering amount of generic animated dvds in the pixar/disney mode.