A preposition. Really and truly. A learned Language Log linguist, Geoffrey Pullum, says so in a post this week. If you don't believe me, go read it yourself.
The short version of the story is that we can blame John Dryden for getting ranty about ending a sentence with a preposition, way back in 1672. Dryden gave no rationale for his judgment, and in fact ended sentences with prepositions in his own writing. A century after Dryden, a grammarian picked up the idea, and gradually other grammarians started to preach it, too. And today, of course, plenty of people who consider themselves educated writers meticulously strive to torture their sentences to put prepositions in highfalutin places, peppering their writing with pointless whiches.
You want to know where you can stick that preposition? You can stick it right at the end of a sentence. Really. It works. Let me show you: "What are you waiting for?" See? Compare that to "For what are you waiting?" Ick. "I couldn't find what you were looking for." Compare what the New Yorker web site will tell you if your search query comes up empty: "I'm sorry I couldn't find that for which you were looking." Eww! (The latter was what spurred Pullum to write his defense of putting prepositions where they naturally belong.) I've always enjoyed the tortured construction, "up with which I will not put"—I forget where I first read that one.