Auto-run Video/Audio 
Just pausing today to hate sites which have a video or audio set to run automatically when a visitor lands on them.
Most well-behaved sites will at least post a cookie which will reduce the chance that should you return to said site the video/audio will blast you again, but you shouldn't bombard your visitor with potentially unwanted media.
This is from a politeness standpoint, as well as (for some viewiers) a bandwidth standpoint.
Do I really need to download 500K of flash garbage to watch you welcome me to your site? I've clicked on it, I'm reading your content, you've hooked me -- don't be so eager to chase me off.
More understandable, yet still annoying, are web bloggers who embed a video into a post which automatically plays.
Today, I'm looking at
this page(warning! auto-run video!!) which showed up in my RSS reader. This one was especially bad, since the video auto-played in my RSS feeder, when I hit the page, and then after I submitted a comment! And irony of ironies, he has a link to a Google Video which doesn't auto-play!