Those hideous web sites with an endless maze of useless links to pages featuring geometric patterns, jumbles of ASCII characters, etc. of varying levels of glaring irritance. I've seen about five different ones now.
I agree with the Emo comment, too.
And ambient music. I once went to a concert where the opening act was like listening to the intro to "Jet Airliner" for a full 45 minutes. A guitarist with an e-bow, playing what seemed like random notes, a guy playing sound effects on a keyboard, and a guy with an assortment of things he banged on or blew into to make noises...and that was it. I was waiting for them to break into a real song, but they never did. They didn't pause, they didn't make any abrupt changes, they never approached a regular beat or a melody...they just went on for the better part of an hour. Thankfully, I woke up in time to join in the confused, polite applause at the end.
I just don't get it.