2001: A Space Odyssey was cut by 20 minutes or so a couple of days after the first screening. Ditto Playtime by Jaques Tati (in fact, I think Paytime was cut by even more).
The Draughtsman's Contract was originally over four hours long (and might have made more sense in the longer version). The Avengers, as someone has alreday noted, was originally substantially longer (indeed moments from a couple of the subsequently clipped scenes may be seen in the trailer)...