Some great films being named here, but not a lot that "present an alternate reality"! 8)
Existenz definitely does, but was on the original list.

(so was Dark city!)
the Big Lebowski and Cool Runnings... what!!?? :shock:
Bladerunner is one of my favourite films (simple yet profound), but doesn't really represent an "alternate reality", though I guess questioning who we are, and wether we are automatons and if it makes a difference is questioning our reality and vaguely examining other possibilities...
I will check out those Carpenter flicks. Nightmare on Elm Street might be a loose fit, too.
Some of Terry Gilliam's flicks are already on the list, but Tideland? I guess it's sort of a fantasy-in-a-girl's-head-to-escape-reality film, like Pan's Labyrinth... I think Wizard of Oz has been mentioned too, but if not, that's another for the list!
Anyway, keep em coming even if you have doubts! The fact that there are fewer and fewer surfacing means we may almost have exhausted the list, but i am sure there are still some gems out there sparkling in some dark corner!
Cheers,
JBArk