I saw the first Percy Jackson movie before I read any of the books, but I did read all the books after. The same is true for the "Cirque du Freak" books. This happens a lot with teen fiction. It's a guilty pleasure, and they're usually very fast and easy reads. (I read a lot of junk, but I still refuse to read or see Twilight, and I won't read 50 Shades)
I read the Hunger Games trilogy after the first movie came out, but before I watched the movie.
I began reading the Harry Potter series around the time Book 5 came out, which was well into the movie series, but I'd seen only the first movie at the time.
I watched the first season of Game of Thrones before I read all the novels.
I read most of Micheal Crichton's books after I watched the movies. My favorite of his books is one of the worst movie adaptations - "Timeline." I still enjoy the movie for what it is, but it was not a great movie.
I didn't know half of Nick Hornby's books WERE books until well after I watched their adaptations (Fever Pitch, High Fidelity, About a Boy).
There are probably a ton I can't recall right now, but those are the ones that stand out to me.