Abusing animals for fun really gets to me. It doesn't always have to be overt though, deliberate neglect is just as bad.
Drivers that harass others on the road. It's worst when the aggressive driver is the one doing the wrong thing like speeding or being erratic, since some of them think they're great drivers, but in reality they're only safe because everyone else if following the rules and being predictable. Aggression to drivers that simply make mistakes annoys me to no end. People can make mistakes, that's why we have following distance.
People that are really ungrateful for what they have. I'll admit I'm really lucky in life; I have a car, a good PC setup and a loving home. I'm not suggesting we have to spend all our time being grateful for everything, but you can appreciate what you've got by taking good care of what you have.
Religious zealots are the worst, particularly when they twist a message to be about hate and cruelty. Westboro Baptist Church is the worst for this. Christianity is about love and tolerance (kinda like My Little Pony), not about justifying aggressive homophobia.