Brick

Film noir set in high school. What's funny is, I've never really been a big fan of noir (though a handful of my favorite movies do qualify), but this movie has sparked my interest in the entire genre.
The dialogue was just freakin' cool. I actually had trouble keeping up and had to turn on the subtitles (I was also at work which hurt). It wasn't even what they were saying, it was just how they were saying it (ie word choice ruling over content).

It wasn't necessarily a great movie, but I always award tons of bonus points for originality and this movie is one of a kind. Go see it.