Preview Movies enables you to watch movie trailers in high definition, by downloading them to your computer. We would recommend using a program called VLC Player to watch the HD trailers.

To save the trailers to your computer, right click on the link, and click "Save Target As..." On certain browsers, remember to rename Yahoo! trailers from *.dll to *.mov.

Trailer Sources:
Apple Trailers
Yahoo Trailers
Comingsoon.net
TheMovieBox.net
Movie-List
IGN Movies