Another interesting and simple ad model that I saw was on a bitcoin gambling site. The ads were a small image banner at the top of the site.
Each advertiser would choose a daily rate to pay. Then each time the page was loaded an ad was selected randomly weighted by daily rate (eg. Alice pay $1/day and Bob pays $2/day, Bob's ads show up twice as often).
I like how simple it was while still allowing advertisers to bid against each other. The total ad revenue of the site would scale directly with website traffic. There are no trackers, no third party services, and no JS. Just simple get what you pay for. Want more? pay more.