Transformers: Dark Side of the Moon
Hulk (Ed Norton version) (sonic cannon scene and the climax battle)
Master and Commander (cannon attack where they shoot off the rudder)
How to Train Your Dragon
Star Trek (recent one directed by JJ Abrams)
iRobot (specifically the car chase through the tunnels while being attacked)
Black Hawk Down
War of the Worlds (Tome Cruise version... not the best movie, but great LFE)
Pearl Harbor (initial attack)
Casino Royale (initial foot chase scene)
If you're geeky enough to have a blu ray burner on your computer (they're under $100 now) there's a guy who put together a blu ray you can download free from torrent sites that has 30+ of the best clips from the movies I've listed and more, all specifically chosen to show off aspects of your HT. Greatest demo disc ever!
I will repeat what just about everyone has already said, Transformers Dark of the Moon will give your system a non stop workout, it is just amazing to watch and listen to especially if you can get past the poor acting and story.
If you are looking to give a sub woofer a workout a great movie that I have not seen mentioned yet is U-571 the scene where they are dropping the depth charges is intense, also Valkyrie has few scenes where there is bombing that will shake your walls, especially in the beginning when Stauffenberg is injured.