I don't know how long this site has been up, or if everyone has heard about this before me, but, holy crap, I love Trailers from Hell. If you're not familiar with this site, it was started by director Joe Dante and features contemporary horror movie creators commenting on old genre movie trailers. Current contributors include Dante, Mary Lambert, John Landis, Sam Hamm, Allan Arkush, Mick Garris, Larry Cohen, and Edgar Wright. Here's Edgar Wright's commentary on the great trailer for Mario Bava's Danger: Diabolik:
(I love the Tor Johnson/Looney Tunes mash-up for their logo.)
The trailer selection is pretty diverse, including B-movies, Hitchcock films, and many non-horror movies. My favorite so far is Joe Dante's commentary on the trailer for The Innocents, which is my single favorite horror movie of all time (though Robert Wise's The Haunting is a close second). Dante makes the point that this sophisticated, psychological ghost story is given a crappy trailer that makes it look like a sensationalized B-horror movie. Here's the link.
You may also, if you so choose, watch the trailers sans commentary.
The site promises to update three times a week, and I couldn't be happier about that.