I have a few sites that I own generating a passive income. Separately they generate a moderate amount each month, but as I have a few it adds up. I have a contact that I submit my sites to for approval. They get sent to an ad network and if they get approved we host some adds on my sites and that's all I do, my contact deals with the rest and they just earn on autopilot. The good thing is the ads can be placed on a subdomain away from the rest of the site. Only issue is getting the sites approved which can be tricky, but once they do it's plain sailing. Sometimes my sites do get taken off the network, but as I don't need to do any work on them once they are approved it's not a major loss and I still have site left that I can do whatever I like with. My plan is to scale up the operation and keep on submitting more and more sites, hopefully I will get a load approved then happy days!