Is there a word to describe someone who tends to disagree with others only to upset them?

What's the word to describe someone who acts arrogantly and always disagrees with others unreasonably in order to upset people around him/her?

[I'm not looking for adjectives like unpleasant, annoying, unfriendly, rude, I'm looking for a more specific term like opinionated, didactic, loquacious, gregarious ]

I think I'd come across it before but I've forgotten what it is..

A curmudgeon is someone who is bad-tempered and disagreeable - so curmudgeonly?

A contrarian is someone who takes an opposing view, especially for the sake of being difficult, contentious or in opposition to the generally held view. This could also be used as an adjective.

A troll is, in a certain context, someone who says something deliberately for the purpose of insulting or upsetting someone. (Thanks RegDwight AAA, and no offense intended to Norwegians).

My British friends and relatives might also say that such a person was "playing silly buggers" (or beggars).

I'd describe someone as antagonistic if they are the type of person that thrives on disagreement and conflict for its own sake. These are the type of people who will start arguments for the sole purpose of creating a tense, adversarial atmosphere.