Perhaps many of you already know but I would like to inform you anyway since I like to wiev trailers as soon as they are uploaded to the web. So I check "Movie-List" and "Apple Quicktime Trailers" web pages nearly everyday.
Sometimes a song or even an instrumental music in the trailer affects me so much that I spend all my day just to find its name.
For this types of situations I would reccomend you http://www.soundtrack.net/ web page. Also you can (if you are interested) search for all details of composers of your famous movies or information about a movies full soundtrack etc.
For trailers, just go to that website and click "Resources", in that page you'll find a section named "Trailer Music" where you can surf on alphabetised movie names.
Hope it helps you