Weird idea i had while in the shower: Make a browser (or even better an addon for commonly used browsers) and find some reasons why people would download and install it. The addon blocks all the ads on the websites and replces them with your own adds. People who don't use adblocker probably don't care whether the website or the browser itself shows the ad. This would be somewhat unethical but apart from that I don't see why this wouldn't work.

If you actually go ahead and do this, feel free to do so but give me 10% or something.