Popups, popunders, etc.
What's the worst of the worst?
To start with, take a look at this annoying ad for the Motorola Vbox on this page:
http://www.canoe.ca/TelevisionLetter..._dave-sun.html
It pops up on every mouseover, even after repeated closing. Closing can only be effected by a close button in a counter-intuitive place. They even place a "more info" + button where the user would expect the Windows x close button.
This ad even pops up when you have Panicware's popup protection activated.
This ad drives me nuts!