My biggest peeve is people who can always get a better deal.
You know the type - I'll buy an MP3 player, for example, and show it to someone. They look at it and say 'Hey, that's cool'. Then they ask how much it was, and I tell them, and no matter what, it's always 'Oh man you got ripped off' or 'I could've got it for you cheaper' or something like that. I bought a pair of earphones last week for 10 bucks, and my flatmate went on and on about how he knows somewhere I could have bought them for 9.
And a similar thing: last year I bought a car and my brother's friend happened to stop by a few days later and wanted to see the car, so I showed him. The first thing out of his mouth was 'Oh yeah, Mistubishis suck, they always break down, you should've bought a Honda'.
God I hate those people