I have told of this site many times now, and i still think it has provided some of the best demo audio i have for my HT. The clips are generally quick, short and blow you away, they are great to play before you pop in a movie:
MultiChannel SR 5.1
Many of the clips are in swedish, so you may want to stay away from them, but the ones to get, the ones i think are awsome and in english are:
The Run
Norway Demo
The Banff Demo
Electronics for beginners
The History of Sound
The car chase (I love this one)
check them out, they're in DTS and you just burn them as an audio CD and play them in your DVD player.
(if you don't mind the swedish talking, the rest of the tracks have some amazing use of surround sound and bass

also check out the Prix Europa Opening Concert for some great DTS music)
then for movies, i like to play a bit of:
Finding Nemo: Whale scene is usually good
Master and Commander: already mentioned
Toy Story 2: the opening sequence
LOTR Return of the King: any parts where those elephant thingis are stomping around.
Titan A.E. the opening scene, and the ice field scene,
U571: already mentioned
Star Wars I: the pod race is excellent
Star Wars II: the opening scene
Black Hawk Down: F***ing Irene scene, with the helicopters, great LFE
Also, something else i have, which, especially for bass and LFE, is a must have:
Chesky's Wierd Absurd Wonderful Surround Show
This disc is strange at first, but it is still quite an experience, and has some of the lowest, long lasting LFE i have seen/heard. The Thunder Stom track usually impresses people often as well
If you also do a search in the
AVS forums and here at Audioholics, you should find may other threads that talk of the same thing . . .
cheers
KJ