I'll start:

- Auto-playing a video when you open it

- Auto-playing a video after you've had it open and sitting quietly in the background for 2 hours (CNN has been doing this a lot recently)

- Forcing you to answer a lame survey question before showing you 90% of the content

- Forums which make you register before you can read them

- Captchas which are nearly impossible to read for humans

- Top 20 lists which force you to click through to the next page for each item

- Redundant links where you have to keep clicking through several pages to do the same thing. (American Airlines is a good example, where you have to click "Book Awards" and then scroll down on the next page to find the link for "Book Award Travel" to redeem your miles.)

- Videos with commercials which are impossible to pause or mute unless you use your computer's volume control