Of all the entries on this list, the ending of United 93 is the only one where I'm rather self-conciously taking the 'kickass' adjective off (it's an amazing scene, but 'kickass' sort of feels inappropriate). I haven't actually watched the movie since Linds and I saw it in theaters. I probably won't watch it again.
I did just watch the ending again, though, and it's as blood freezing and haunting as ever.
...Just Outside of Shanksville, Pennsylvania
There were brief moments in United 93 where I forgot - or maybe purposefully lost track of the fact - that the on screen happenings were based on actual events. The leadup to the passenger's final assault on the cockpit was one of those moments.
The actual attack is messy and disorganized, but the captives finally break into the cockpit and attempt to take back control. Then the camera pans away from the fight and out the window at the rapidly approaching ground below.
The final seconds of the film are surprising in their suddenness. I watched the clip below a few times in order to get a proper feel for a writeup, and every time, it caught me at least a little unawares. The black screen hangs on for a few extra seconds to really drive the point home.
The last fifteen minutes of the movie are here. Skip ahead to ten minutes to watch the final scene.