For me the only reason that can make someone angry is plans getting screwed up. And by plans I don't mean anything complicated. Let me present a few examples:
You sit on your pc. Without having given it much thought really, because this is something that you do usually and doesn't require any "planning" you "plan" to spend some time using it for a number of reasons. Work, games, surfing, chat or whatever. Suddenly, your pc breaks down. Nerd Rage follows.
You heard from your friends that they'll be visiting you today. Therefore, even though you have some work to do, you know that when your friends come, you'll leave it incomplete, making it difficult to continue afterwards so you "plan" to have fun all day today and concentrate on the thing all day tomorrow. Two hours before your friends are about to come, you are already in a let's-party mood, being super lazy and all that, you receive a call from a friend, telling you that something came up, so they'll visit you tomorrow instead. This destroys your "plans" you have no motivation for working, and you have wasted TWO days instead of one. Work-related Rage follows.
You're playing a video game. You have made it pretty far, know the drill and are ready for everything. You have semi-mastered the controls and the way the game thinks and continue on cheerfully. Suddenly you die. In your mind, you had already made some "plans" like - I'll go a bit further and then close it - or - I'm actually going pretty good, I want a challenge, heh! - or stuff like that. All these thoughts crumble instantly when you die. Gamer's Rage.
The same thing can also be explained when you have certain expectations of people/situations and the people/situations don't live up to what you wanted/expected thus annoying you => making you angry. For this theory can explain every sort of anger that I have ever lived through. Be it from a serious situation or a game. I presented light problems on purpose though.
So, counter against those? Be open minded. Nothing ever goes the way you think it will. The more you stress out thinking/planning something the greater the strain when it fails. Be simple. Live in a whim. And never expect certain stuff of other people or situations. Other people are not you, and will never live up to your expectations because they're different people and have their own ways. Situations are unpredictable since they can be influenced by millions of stuff.
That's what I (try) to do. I'm not sure if it'll work for anyone else though. Everyone has his/her own way. However this "answer" of mine, is to avoid even the possibility of being angry. Doesn't always work of course. If it fails, do some stuff you love. A good slice of pizza followed by a hot cup of tea, a good bath and some book do the trick for me after super-rageXD
I kinda wrote too much again.