>I've come to decide that the only ads I consider "legit" are where the site owner strikes a deal with another business that is interested in advertising on their site, the site owner hosts the ad on their own server, as a picture banner or text or perhaps a nice block in a side column that says "sponsored content" or whatever, and just links to the other business.

I agree. When it comes to ads for niche content (blogs, forums, etc). The ad industry sells online ads like they're TV commercials but the companies buying the ads should be looking at them like partially sponsoring a race team in exchange for your logo showing up in front of people who are interested in your type of products/services.