A long time ago back when browsers supported Java applets you could include applets in your banner ads. I worked at a place that had a product that included a chat service. The chat client was in Java and would have been quick and easy to convert into an applet.

I tried to convince the person in charge of our advertising that we should include such a chat applet in one of our banner ads, which would put everyone currently viewing the banner into an anonymous chatroom. Unfortunately he wouldn't go for it.