Reel Classics provides downloadable access to movie trailers and clips for films dating as far back as 1920. Select AV Gallery to explore.
Jaman features trailers for many international titles, but does not include materials pre-1960.
Turner Classic Movies has an online collection of trailers for the films played on the channel. Pay special attention to make sure you are viewing the "Original Trailer"-- TCM also contains re-issued and video-release trailers, which may differ significantly from the original.
MGM.com hosts trailers from most MGM releases. Browse the title lists, and look under Multimedia on each title's page to view trailers.