Ask HN: What's up with this type of superannoying ads?

http://davidadewumi.com/2008/07/06/why-are-black-nerds-so-unpopular/

On this site and a lot of others, most often serious bloggers, there are really annoying ads.

Insetad of having them in a nice column on the side some #"#¤#¤ decided it is a good idea to have them pop up when point the mouse to a special word.

I fidn this to be the most annoying from of ad ever.

It is very intrusive and makes me close the site immediately.

It makes some sites unreadable.

Is it the html so it is the site-designers fault or is it some sort of special ad-network?