There is a WONDERFUL new website on the net called Trailers From Hell! featuring movie trailers from all these old classic movies, most of them grindhouse or horror flicks. But along with the original trailers, they have commentary from some of today's most celebrated directors and other movie insiders talking about both the movies and the trailers.
This is popcorn viewing at its finest. (You can watch the trailers with or without commentary).
Here's an example featuring "The Attack of the 50 Foot Woman" with commentary by Joe Dante.
I also love this commentary by John Landis on a music concert movie called "The T.A.M.I. Show" where Lesli Gore is the biggest star on a bill with The Rolling Stones, the Beach Boys, James Brown and Smokey Robinson.