That Sound in Trailers
Inception (2010) is the first film I noticed it in. It's pretty effective, brings a sort of dread. I can only think of three trailers right now, but I have no doubt in my mind that there are (were) many others. It's funny how these things catch on (if you're wondering what 'that' sound is, it's the deep note repeated. In these three trailers, it all kicks in towards the end).
UPDATE - After posting this on Facebook a few of my friends came forth with suggestions, the best of which was the Inception Horn. Sounds legit. I Googled, and came across this short article (among others) saying how 2011 was the year of Inception-like-trailers, i.e. had that horn in it. So there you go.