Based on the quite small number of American independent films I've seen, here's the supposed masterpieces. I've cut out those films that are mainly made in other countries than USA, but are partly produced there.
Feel free to give me any suggestions. Please, do correct me if I'm completely lost with these picks. I'm sure that many films are missing here, even those I've seen many times.
Nice and very intresting list by the way. I suppose that Sideways and Spellbound are two considerable choices on your list. Also I Heart Huckabees is one contingent choice, but of course feel free to choose. ;)