Premium plans, or non-tracking ads (that are personalized based on the content you see instead of your profile). Read The Docs does that - they have a single,

Premium plans, or non-tracking ads (that are personalized based on the content you see instead of your profile). Read The Docs does that - they have a single, static ad spot (just an image and text, no JS) and they’re chosen based on the content of the page you see (so instead of tracking you, they’re tracking public content, which is totally fine by me). They also seem to have only quality ads, I haven’t seen any shit or spam there ever, presumably because they’re human-reviewed. You should do the same to make sure the ads actually have some value (if you let shit ads through your users will just learn to tune them out if they can’t adblock them, and you’ll loose out either way).