Back in the day, I used to belong to a forum for a program that allowed you to connect to the privately run Napster servers; the owner of the forum complained that he'd need to start running ads soon, to help him make some money to cover expenses. In theory, I did not object to this, since the web-based "community" that we had was pretty cool, and I enjoyed the interactions there.
That all stopped when we were subjected to the first ad; a Doritos ad that scrolled up from the bottom of the screen, complete with a very loud, obnoxious sound of a toilet flushing.
*boggle*
This single action resulted in my discovery of the Mozilla project in 2002; I have not used Internet Explorer for general web browsing since then. Thanks, napigator.com!