I finally experienced this little phenomenon yesterday and it was definitely trippy.
My husband told me about the Dark Side of The Rainbow and I had to see it for myself before I believed it. It's when you watch the Wizard of Oz movie with the sound muted and play Pink Floyd's Dark Side of the Moon at the same time. You have to hit the play button the third time the MGM Lion roars at the beginning of the movie, and the music synchronizes with what's going on in the film.
I found out that it's not just hype. It's really cool! And I was wondering how this ever got by me. I mean, I used to smoke pot, so why did I never hear about this? What pisses me off is that V was the one who told me about it too, even though I like to think of myself as wearing the "useless-trivia-about-pop-culture" pants in the family. So everyone knew about this but me. That sucks. But, I am glad I finally saw it. You don't even have to be stoned to get the trippy effect of it either, although it probably helps (we weren't high when we watched it).
How did anyone discover this? I figure it had to be some stoners who just happened to hit the play button at the right time (because, seriously, who else watches the Wizard of Oz and listens to Pink Floyd at the same time?) Bless those stoners, whoever they may be.
I googled this and there are a zillion sites about it. Some include a list of synchronizations, but don't read them if you haven't seen it yet. Watch the movie, play the album, and look for them yourself. It's more fun that way. And even though I am an enemy of Wikipedia (ever since they banned me), they do have a pretty good entry on it.
People argue about whether Pink Floyd purposely aligned it with the movie and I think that's a bit of a stretch. It's just a very strange coincidence and one of those mysterious things that makes rock music fun.