I never knew that SO displayed ads until today. A few minutes ago I opened the Stack Overflow website in IE7 and was surprised to see an ad banner on top of a question. I've been using Chrome for over a year and never needed to move back to IE.
In Chrome, I don't have any extensions like AdBlocker or something installed on my system, so why haven't I ever seen ad banners on top of questions before? Do the developers themselves detect the browser and shows ad accordingly or what? What's the story behind this?