"I like the one about Raiders of the Lost Ark where Indy shoots the sword fighter. He was supposed to fight him in a shoot that would've taken 3 days. However Ford was really sick from dysentery and so convinced Spielberg to do it this way."

"This is a smaller one than the plethora of LOTR info already here, but still one of my favorites:

In the movie "The Holiday", there is a scene in a video rental store where Miles (Jack Black) is introducing Iris (Kate Winslet) to a variety of famous music from movies - Chariots of Fire, Gone With The Wind, etc. When he gets to "The Graduate", he starts singing "Mrs. Robinson", and the camera cuts over to Dustin Hoffman, shaking his head at their conversation.

Dustin Hoffman's cameo was completely unplanned - he knew the director, saw a bunch of equipment at the Blockbuster, stopped in to say hi, and they wrote his cameo in on the spot."