Found this browsing another forum and thought I'd share. I did a quick search and couldn't find this posted here before so hopefully no repost... These seem to be freely available so hopefully not breaking any rules either :)
The website has an archive of HD film studio trailers and things like THX and Dolby Digital trailers as well as those HD videos they use to demo HDTVs in stores. There are heaps of them and all are in 1080p and most in Dolby 5.1.
So if you want to give your HDTV a workout, impress some mates or just always wanted a copy of those HD videos then here they are. Click on the part numbers to download.
(Mods feel free to move to another forum, I just put it here as they are downloadable computer files so most likely to be played on an HTPC)