In no particular order, these are three uniquely American developments pretty much from the last century.
1). Collegiate marching bands
2). Black gospel choirs
Obviously, these three owe a lot to what has gone on before, as I'll explain.
As far as I know, the modern university marching band is only seen in the United States. Largely because we're the only ones who play American football, perhaps. They do stem from military bands that long precede the modern marching band, especially from places like the UK. The bands really add to the atmosphere of the football game, and some are worth going to see just for the bands themselves (examples: Ohio State's The Best Damn Band In The Land [TBDBITL], Texas A&M's Fighting Aggie Band, and Florida State's band). Seeing 200 to 400 people on the field doing all coordinated playing together ... it's pretty cool. Even though lots of people use the half to go pee or get a hot dog. And there are few things like hearing Amazing Grace performed by a band.
Black gospel choirs
If you've never been to a black church you may not understand what I mean and I probably can't explain it that well, but the atmosphere sometimes can get really charged up. It comes from Africa and historically from the spirituals that slaves used to sing (field hollars are now call and response music), but when they get going you're going to WORSHIP, Christian or non-Christian. It's also kind of a Southern thing, especially revivals. Not limited to the US, of course.
Obviously a genre of music, not a specific kind of group. Jazz is far too complex for me to be able to explain well, but I can say that I learned more about music by being in jazz band for four years in high school than I ever did in regular band.
But don't get me started on how sad it is that 50 cent is as popular as he is.