1. When Luke shoots a stormtrooper from the unextended bridge and he falls down the chasm in ANH.
2. When Chewie throws stormtroopers in the carbon-freezing chamber in ESB.
3. When the Weequay is thrown into the sarlaac pit in ROTJ.
4. When a Naboo pilot gets shot running to his ship right before the attack on the droid control ship in TPM.
5. When Padme's ship explodes when it arrives in Coruscant in AOTC.
And let's not forget it's appearence in other movies:
In Raiders of the Lost Ark: When the Nazi's have the ark in a truck and Indiana Jones tries to take the truck, they accelerate fast, a Nazi falls out of the truck onto the hood of the car, he screams.
In Lord of the Rings: The Two Towers: When a guy falls off the wall in the Battle at Helm's Deep.
check out this site about the Wilhelm Scream: Ledgend of the Wilhelm Scream