All they would have to do is throw affiliate links to shit they use in the footer.
Domain Registration Provided By:
GoDaddy.com
Web Hosting/Servers Provided By:
etc...
not really advertising if you're only promoting shit you actually use and are happy with. I do this on a few sites and make referrals and with their page views a few footer text links wouldn't kill anyone and would probably do quite well. Waiting on my 10% consulting fee.