One of the coolest things about the Xbox 360 is the Xbox Live Marketplace. From there, you can download game demos, theme packs, gamer pictures, and more, including game and movie trailers AND movies and TV shows! Since I have the hard drive for my console, I can download tons of stuff to it (whether from the Marketplace or from another computer on my network). So, I've spent a couple hours checking out games and videos.
The movie section is pretty limited having only 94 titles. But, every movie comes with a trailer (which you can watch for free, of course). I noticed that they have the first six Star Trek movies in their library, so I started watching the trailers. Now, being a Star Trek fan, I'm pretty familiar with all of the trailers, and I know every movie pretty much by heart. It's interesting, though, to see how crappy the trailers were back in the late 70s and 80s and how much they got better once the 90s rolled around.
You don't need an Xbox 360 to check it out, though. Go to a site like IMDB and look at movies from the 70s, 80s, and 90s--most of them should have trailers. You'll see what I'm talking about pretty much instantly...it's very obvious and borderline hilarious.