The most well known places are of course YouTube, and Apple's trailer site, but here are some alternatives if you want high quality downloads and/or surround sound versions.
Surround sound versions are especially useful if you want to edit mashups or videos, because as explained in my tutorial you can isolate the center channel, and hear versions of trailers like the one that got accidentally released for the Tom Cruise reboot of The Mummy.
HD-trailers.net - Regularly updated, with pages dedicated to each film with all trailers in the campaign.
The Digital Theater - Rotating selection of trailers with high quality MP4 and MKV files with AC3 or uncompressed audio. Many, but not all have true discrete 5.1 channels with isolated dialogue in the center channel.
Demo World - Surround sound encodes of trailers from demo discs.
Movie-list.com - Encodes classic trailers, and has an active forum with trailer news and high quality encodes.
Game Trailers YouTube - Latest video game trailers
@Microtrailers - A bot that takes 6 random seconds from the new releases on Steam. More of a fascinating look into how quickly you can decide you find something appealing or not.