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