I've got a new one.
People who repeatedly tell you 'amusing' stories that don't involve anyone you know whatsoever and then expect you to laugh. These are the people that don't seem to have a grasp of the concept of 'you had to be there'. I'll give you an example.
"Oh, I'll tell you something really funny. Michael fell asleep yesterday and Lucy was like 'POKE HIM!', so I did!"
HA. HA. HA. HA. HA. Oh stop it, you're killing me. No seriously.
What's worse is when they also feel the need to inform everyone else of the same story right in front of you, so you have to pretend you're amused another 18 times that same morning.
Another one is when they do that right in the middle of a conversation you're having with someone else, and thereby instantly kill it. I was chatting to one of my friends who's just passed her test about what cars she's been looking at and likes etc (to put this in context, she's spending around £5k), and then this other girl inexplicably hijacked the conversation to talk about the wonder that is the Fiat Uno (how it's got 'a load' of leg room, how her dad's one overtook a Porsche once...), and then somehow subtly changed the subject to something or other about what some random friend did at some weird RAF camp she goes to. I mean WTF?