People who refuse to use the english language as it was originally intended. Now, I am not a grammar or spelling nazi by far: I don't use perfect English by any means, but at least I try.
a) Foreigners who come to this country to make a better life for themselves, but refuse to speak english when they are out in public. Now, if they just got here, and haven't learned the language yet but are trying, good job, keep working on it. But when they speak relatively proper english but refuse to speak it when their friends are around, that make me turn red with anger. Where I work, I get alot of middle easterners that come in looking for help. They usually bring their friends or family, and ask me for assistance in English. Then, once I've answered their questions to their satisfaction, they suddenly starts talking in Indian/Pakistani/whatever they speak. They give me no notification that they don't need my help any longer, instead they just rudely start speaking to each other in their native language. The other day, it happened again. They asked me about memory cards and how many pictures can be held on a "one Gee Bee" card (I correct them and tell them it's called a Gigabyte). As soon as I told them the answer, they started talking to each other in their language, and I just huffed and walked away. They called out to me and asked why I was leaving them, and I turned and replied in these exact words: "Because it's extremely rude for you to speak to your friend or brother in another language when I am trying to help you, and I don't appreciate that." Another co worker wound up finishing up with them, but they didn't say anything about it.
b) People who use ridiculous shortenings of words online, especially in IMs or forums. The letters "u", "b", "y", and "o" are NOT WORDS. DON'T USE THEM LIKE THEY ARE. "haha o u got pwn3d y0, y u gonna let him do that 2 u?" That kind of shit makes me want to punch the moron's lights out for being a retard.
How about everyone else? What are your pet peeves?