Most of you beat me to all the Hollywood ones I was thinking of. The Calico, the Mauser, and whatever they used for the Blade Runner piece. But there is also the Bushmaster M17S. It gets used in a lot of sci-fi flicks. I think in an SNL, Chris Farley is running to the space ship with one while his pants are falling off! Seen it used in other movies too, but I can't be positive of the names so I'll leave them out. They seldom do much to the Bushmaster bullpup either.
And before you think of getting one, I had one back in the 90's. Bad trigger, but it also broke after a few hundred rounds. Sent it back, they fixed it, got a crack in the same place. I guess it is the reason they aren't popular, but because of that, they make a great sci-fi weapon!